diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index 442ea23..2d80b69 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -1,6 +1,5 @@
-#Fri Nov 14 16:00:10 PST 2014
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.8.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.2-bin.zip
diff --git a/gradlew b/gradlew
index 91a7e26..cccdd3d 100755
--- a/gradlew
+++ b/gradlew
@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 ##############################################################################
 ##
@@ -6,20 +6,38 @@
 ##
 ##############################################################################
 
-# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
-DEFAULT_JVM_OPTS=""
+# Attempt to set APP_HOME
+# Resolve links: $0 may be a link
+PRG="$0"
+# Need this for relative symlinks.
+while [ -h "$PRG" ] ; do
+    ls=`ls -ld "$PRG"`
+    link=`expr "$ls" : '.*-> \(.*\)$'`
+    if expr "$link" : '/.*' > /dev/null; then
+        PRG="$link"
+    else
+        PRG=`dirname "$PRG"`"/$link"
+    fi
+done
+SAVED="`pwd`"
+cd "`dirname \"$PRG\"`/" >/dev/null
+APP_HOME="`pwd -P`"
+cd "$SAVED" >/dev/null
 
 APP_NAME="Gradle"
 APP_BASE_NAME=`basename "$0"`
 
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS=""
+
 # Use the maximum available, or set MAX_FD != -1 to use that value.
 MAX_FD="maximum"
 
-warn ( ) {
+warn () {
     echo "$*"
 }
 
-die ( ) {
+die () {
     echo
     echo "$*"
     echo
@@ -30,6 +48,7 @@ die ( ) {
 cygwin=false
 msys=false
 darwin=false
+nonstop=false
 case "`uname`" in
   CYGWIN* )
     cygwin=true
@@ -40,31 +59,11 @@ case "`uname`" in
   MINGW* )
     msys=true
     ;;
+  NONSTOP* )
+    nonstop=true
+    ;;
 esac
 
-# For Cygwin, ensure paths are in UNIX format before anything is touched.
-if $cygwin ; then
-    [ -n "$JAVA_HOME" ] && JAVA_HOME=`cygpath --unix "$JAVA_HOME"`
-fi
-
-# Attempt to set APP_HOME
-# Resolve links: $0 may be a link
-PRG="$0"
-# Need this for relative symlinks.
-while [ -h "$PRG" ] ; do
-    ls=`ls -ld "$PRG"`
-    link=`expr "$ls" : '.*-> \(.*\)$'`
-    if expr "$link" : '/.*' > /dev/null; then
-        PRG="$link"
-    else
-        PRG=`dirname "$PRG"`"/$link"
-    fi
-done
-SAVED="`pwd`"
-cd "`dirname \"$PRG\"`/" >&-
-APP_HOME="`pwd -P`"
-cd "$SAVED" >&-
-
 CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
 
 # Determine the Java command to use to start the JVM.
@@ -90,7 +89,7 @@ location of your Java installation."
 fi
 
 # Increase the maximum file descriptors if we can.
-if [ "$cygwin" = "false" -a "$darwin" = "false" ] ; then
+if [ "$cygwin" = "false" -a "$darwin" = "false" -a "$nonstop" = "false" ] ; then
     MAX_FD_LIMIT=`ulimit -H -n`
     if [ $? -eq 0 ] ; then
         if [ "$MAX_FD" = "maximum" -o "$MAX_FD" = "max" ] ; then
@@ -114,6 +113,7 @@ fi
 if $cygwin ; then
     APP_HOME=`cygpath --path --mixed "$APP_HOME"`
     CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
+    JAVACMD=`cygpath --unix "$JAVACMD"`
 
     # We build the pattern for arguments to be converted via cygpath
     ROOTDIRSRAW=`find -L / -maxdepth 1 -mindepth 1 -type d 2>/dev/null`
@@ -154,11 +154,19 @@ if $cygwin ; then
     esac
 fi
 
-# Split up the JVM_OPTS And GRADLE_OPTS values into an array, following the shell quoting and substitution rules
-function splitJvmOpts() {
-    JVM_OPTS=("$@")
+# Escape application args
+save () {
+    for i do printf %s\\n "$i" | sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/' \\\\/" ; done
+    echo " "
 }
-eval splitJvmOpts $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS
-JVM_OPTS[${#JVM_OPTS[*]}]="-Dorg.gradle.appname=$APP_BASE_NAME"
+APP_ARGS=$(save "$@")
+
+# Collect all arguments for the java command, following the shell quoting and substitution rules
+eval set -- $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS "\"-Dorg.gradle.appname=$APP_BASE_NAME\"" -classpath "\"$CLASSPATH\"" org.gradle.wrapper.GradleWrapperMain "$APP_ARGS"
+
+# by default we should be in the correct project dir, but when run from Finder on Mac, the cwd is wrong
+if [ "$(uname)" = "Darwin" ] && [ "$HOME" = "$PWD" ]; then
+  cd "$(dirname "$0")"
+fi
 
-exec "$JAVACMD" "${JVM_OPTS[@]}" -classpath "$CLASSPATH" org.gradle.wrapper.GradleWrapperMain "$@"
+exec "$JAVACMD" "$@"
diff --git a/gradlew.bat b/gradlew.bat
index aec9973..e95643d 100644
--- a/gradlew.bat
+++ b/gradlew.bat
@@ -8,14 +8,14 @@
 @rem Set local scope for the variables with windows NT shell
 if "%OS%"=="Windows_NT" setlocal
 
-@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
-set DEFAULT_JVM_OPTS=
-
 set DIRNAME=%~dp0
 if "%DIRNAME%" == "" set DIRNAME=.
 set APP_BASE_NAME=%~n0
 set APP_HOME=%DIRNAME%
 
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS=
+
 @rem Find java.exe
 if defined JAVA_HOME goto findJavaFromJavaHome
 
@@ -46,10 +46,9 @@ echo location of your Java installation.
 goto fail
 
 :init
-@rem Get command-line arguments, handling Windowz variants
+@rem Get command-line arguments, handling Windows variants
 
 if not "%OS%" == "Windows_NT" goto win9xME_args
-if "%@eval[2+2]" == "4" goto 4NT_args
 
 :win9xME_args
 @rem Slurp the command line arguments.
@@ -60,11 +59,6 @@ set _SKIP=2
 if "x%~1" == "x" goto execute
 
 set CMD_LINE_ARGS=%*
-goto execute
-
-:4NT_args
-@rem Get arguments from the 4NT Shell from JP Software
-set CMD_LINE_ARGS=%$
 
 :execute
 @rem Setup the command line
diff --git a/src/main/java/learnrxjava/ComposableListExercises.java b/src/main/java/learnrxjava/ComposableListExercises.java
index a13940c..0db89bb 100644
--- a/src/main/java/learnrxjava/ComposableListExercises.java
+++ b/src/main/java/learnrxjava/ComposableListExercises.java
@@ -17,12 +17,12 @@
 import learnrxjava.types.Video;
 
 /**
- * Mastering concurrency is challenging, But we can make it much easier by simply choosing the right 
- * abstraction to model an asynchronous operation, and then using simple composition operations to 
+ * Mastering concurrency is challenging, But we can make it much easier by simply choosing the right
+ * abstraction to model an asynchronous operation, and then using simple composition operations to
  * glue different instances of these abstractions together to build solutions to complex problems.
- * 
- * To learn how stream composition works, we will first learn how to use the composition methods 
- * (map, filter, flatMap, reduce, zip) to compose together a data structure with which most developers 
+ * <p>
+ * To learn how stream composition works, we will first learn how to use the composition methods
+ * (map, filter, flatMap, reduce, zip) to compose together a data structure with which most developers
  * are already familiar: a list.
  */
 public class ComposableListExercises<T> extends ArrayList<T> implements ComposableList<T> {
@@ -92,11 +92,11 @@ public static ComposableList<JSON> exercise3() {
                 new Video(675465, "Fracture", 5.0));
 
         ComposableListExercises<JSON> videoAndTitlePairs = new ComposableListExercises<JSON>();
-        
+
         newReleases.forEach(video -> {
-           videoAndTitlePairs.add(json("id", video.id, "title", video.title));
+            videoAndTitlePairs.add(json("id", video.id, "title", video.title));
         });
-        
+
         return videoAndTitlePairs;
     }
 
@@ -121,19 +121,12 @@ If our ComposableList had a map() method, it would make projections easier.
     public <R> ComposableList<R> map(Function<T, R> projectionFunction) {
         ComposableListExercises<R> results = new ComposableListExercises<R>();
         this.forEach(itemInList -> {
-            // ------------ INSERT CODE HERE! ----------------------------
-            // Apply the projectionFunction to each item in the list and add
-            // each result to the results list.
-            // Note that you can apply a projectionFunction to a value like this:
-            //  projectionFunction.apply(5)
-            // ------------ INSERT CODE HERE! ----------------------------
-            
+            results.add(projectionFunction.apply(itemInList));
         });
-        
-        //return results;
-        throw new UnsupportedOperationException("Not implemented yet.");
+
+        return results;
     }
-    
+
     /*
     Exercise 5: Use map() to project a ComposableList of videos into a stream of {id,title} JSON
 
@@ -142,14 +135,12 @@ public <R> ComposableList<R> map(Function<T, R> projectionFunction) {
     */
     public static ComposableList<JSON> exercise5() {
         ComposableListExercises<Video> newReleases = ComposableListExercises.of(
-            new Video(70111470, "Die Hard", 4.0),
-            new Video(654356453, "Bad Boys", 5.0),
-            new Video(65432445, "The Chamber", 4.0),
-            new Video(675465, "Fracture", 5.0));
-         
-        // complete this expression 
-        // return newReleases.map(video -> 
-        throw new UnsupportedOperationException("Not implemented yet.");
+                new Video(70111470, "Die Hard", 4.0),
+                new Video(654356453, "Bad Boys", 5.0),
+                new Video(65432445, "The Chamber", 4.0),
+                new Video(675465, "Fracture", 5.0));
+
+        return newReleases.map(video -> json("id", video.id, "title", video.title));
     }
 
     /*
@@ -163,39 +154,38 @@ Use forEach() to loop through the videos in the newReleases list and, if a video
      */
     public static ComposableList<Video> exercise6() {
         ComposableListExercises<Video> newReleases = ComposableListExercises.of(
-            new Video(
-                    70111470,
-                    "Die Hard",
-                    4.0
-            ),
-            new Video(
-                    654356453,
-                    "Bad Boys",
-                    5.0
-            ),
-            new Video(
-                    65432445,
-                    "The Chamber",
-                    4.0
-            ),
-            new Video(
-                    675465,
-                    "Fracture",
-                    5.0
-            ));
+                new Video(
+                        70111470,
+                        "Die Hard",
+                        4.0
+                ),
+                new Video(
+                        654356453,
+                        "Bad Boys",
+                        5.0
+                ),
+                new Video(
+                        65432445,
+                        "The Chamber",
+                        4.0
+                ),
+                new Video(
+                        675465,
+                        "Fracture",
+                        5.0
+                ));
 
         ComposableListExercises<Video> highRatedVideos = new ComposableListExercises<Video>();
 
         newReleases.forEach(video -> {
-            // Insert code here that adds a video to the highRatedVideos list
-            // if it has a rating of 5.0.
-
+            if (video.rating == 5) {
+                highRatedVideos.add(video);
+            }
         });
-        
-        // return highRatedVideos;
-        throw new UnsupportedOperationException("Not implemented yet.");        
+
+        return highRatedVideos;
     }
-    
+
     /*
     Exercise 7: Implement filter()
 
@@ -215,17 +205,12 @@ A Predicate is a test function that returns true or false (ex. x -> x > 1).
     public ComposableList<T> filter(Predicate<T> predicateFunction) {
         ComposableListExercises<T> results = new ComposableListExercises<T>();
         this.forEach(itemInList -> {
-            // ------------ INSERT CODE HERE! ----------------------------
-            // Apply the predicateFunction to each item in the list. If the
-            // result is true, add the result to the results list.
-            // Note: you can apply the predicateFunction to a value like this:
-            // predicateFunction.test(5)
-            // ------------ INSERT CODE HERE! ----------------------------
-
+            if (predicateFunction.test(itemInList)) {
+                results.add(itemInList);
+            }
         });
 
-        // return results;
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return results;
     }
 
     /*
@@ -238,35 +223,30 @@ The map() and filter() functions let us express what data we want without requir
      */
     public static ComposableList<Integer> exercise8() {
         ComposableListExercises<Video> newReleases
-            = ComposableListExercises.of(
-                    new Video(
-                            70111470,
-                            "Die Hard",
-                            4.0
-                    ),
-                    new Video(
-                            654356453,
-                            "Bad Boys",
-                            5.0
-                    ),
-                    new Video(
-                            65432445,
-                            "The Chamber",
-                            4.0
-                    ),
-                    new Video(
-                            675465,
-                            "Fracture",
-                            5.0
-                    ));
-
-        // ------------ INSERT CODE HERE! -----------------------------------
-        // Chain the filter and map functions to select the id of all videos
-        // with a rating of 5.0.
-        // ------------ INSERT CODE HERE! -----------------------------------
-        //return newReleases // Complete this expression
-
-        throw new UnsupportedOperationException("Not implemented yet.");
+                = ComposableListExercises.of(
+                new Video(
+                        70111470,
+                        "Die Hard",
+                        4.0
+                ),
+                new Video(
+                        654356453,
+                        "Bad Boys",
+                        5.0
+                ),
+                new Video(
+                        65432445,
+                        "The Chamber",
+                        4.0
+                ),
+                new Video(
+                        675465,
+                        "Fracture",
+                        5.0
+                ));
+
+        return newReleases.filter(video -> video.rating == 5)
+                .map(video -> video.id);
     }
 
     /*
@@ -283,27 +263,27 @@ Chaining together map() and filter() gives us a lot of expressive power. These h
      */
     public static ComposableList<Integer> exercise9() {
         ComposableListExercises<MovieList> movieLists = ComposableListExercises.of(
-            new MovieList(
-                "New Releases",
-                ComposableListExercises.of(
-                        new Video(70111470, "Die Hard", 4.0),
-                        new Video(654356453, "Bad Boys", 5.0))),
-            new MovieList(
-                "Dramas",
-                ComposableListExercises.of(
-                        new Video(65432445, "The Chamber", 4.0),
-                        new Video(675465, "Fracture", 5.0))));
+                new MovieList(
+                        "New Releases",
+                        ComposableListExercises.of(
+                                new Video(70111470, "Die Hard", 4.0),
+                                new Video(654356453, "Bad Boys", 5.0))),
+                new MovieList(
+                        "Dramas",
+                        ComposableListExercises.of(
+                                new Video(65432445, "The Chamber", 4.0),
+                                new Video(675465, "Fracture", 5.0))));
 
         ComposableListExercises<Integer> allVideoIdsInMovieLists = new ComposableListExercises<Integer>();
 
-        // ------------   INSERT CODE HERE!  -----------------------------------
-        // Use two nested forEach loops to flatten the movieLists into a list of
-        // video ids.
-        // ------------   INSERT CODE HERE!  -----------------------------------
-        //return allVideoIdsInMovieLists;
-        throw new UnsupportedOperationException("Not implemented yet.");
+        movieLists.forEach(movieList -> {
+            movieList.videos.forEach(video -> {
+                allVideoIdsInMovieLists.add(video.id);
+            });
+        });
+        return allVideoIdsInMovieLists;
     }
-    
+
     /*
     Exercise 10: Implement concatMap()
 
@@ -326,25 +306,17 @@ public static ComposableList<Integer> exercise9() {
     is equivalent to
 
     ComposableList.of(0, 10, 20, 100, 110, 120, 200, 210, 220)
-     */   
+     */
     public <R> ComposableList<R> concatMap(
-        Function<T, ComposableList<R>> projectionFunctionThatReturnsList) {
+            Function<T, ComposableList<R>> projectionFunctionThatReturnsList) {
         ComposableListExercises<R> results = new ComposableListExercises<R>();
         for (T itemInList : this) {
-            // ------------ INSERT CODE HERE! ----------------------------
-            // Apply the projection function to each item in the list.
-            // This will create _another_ list. Then loop through each
-            // inner list and add each item to the results list.
-            // Note that you can apply a projectionFunction to a value like this:
-            // projectionFunctionThatReturnsList.apply(5)
-            // ------------ INSERT CODE HERE! ----------------------------
-
+            projectionFunctionThatReturnsList.apply(itemInList).forEach(results::add);
         }
-        
-        //return results;
-        throw new UnsupportedOperationException("Not implemented yet.");
+
+        return results;
     }
-    
+
     /*
     As you may have already guessed, the ComposableList class has the concatMap method. The concatMap method 
     may seem pretty abstract at first, and it may not be immediately obvious how it can be used to transform 
@@ -367,18 +339,11 @@ public static ComposableList<Integer> exercise11() {
                                 new Video(65432445, "The Chamber", 4.0),
                                 new Video(675465, "Fracture", 5.0))));
 
-        // ------------   INSERT CODE HERE!  -----------------------------------
-        // Use map and concatAll to flatten the movieLists in a list of video ids.
-        // return movieLists // finish expression
-        // ------------   INSERT CODE HERE!  -----------------------------------
-        // **************ANSWER START***************//
-        // return movieLists.
-        //     concatMap(movieList -> 
-        //         movieList.videos.map(video -> video.id));
-        // **************ANSWER END***************//
-        throw new UnsupportedOperationException("Not implemented yet.");
+
+        return movieLists
+                .concatMap(movieList -> movieList.videos.map(video -> video.id));
     }
-    
+
     /*
     exercise 12: Retrieve id, title, and a 150x200 box art url for every video
 
@@ -394,51 +359,51 @@ see if you can solve this problem using _only_ map(), concatMap(), and filter().
     */
     public static ComposableList<JSON> exercise12() {
         ComposableListExercises<MovieList> movieLists = ComposableListExercises.of(
-            new MovieList(
-                "Instant Queue",
-                ComposableListExercises.of(
-                    new Video(
-                        70111470,
-                        "Die Hard",
-                        5.0,
-                        null,
-                        ComposableListExercises.of(
-                            new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/DieHard150.jpg")
-                        )),
-                    new Video(
-                        654356453,
-                        "Bad Boys",
-                        4.0,
-                        null,
-                        ComposableListExercises.of(
-                            new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys200.jpg"),
-                            new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys150.jpg")
-                        ))
-                )
-            ),
-            new MovieList(
-                "New Releases",
-                ComposableListExercises.of(
-                    new Video(
-                        65432445,
-                        "The Chamber",
-                        4.0,
-                        null,
+                new MovieList(
+                        "Instant Queue",
                         ComposableListExercises.of(
-                            new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber150.jpg")
-                        )),
-                    new Video(
-                        675465,
-                        "Fracture",
-                        5.0,
-                        null,
+                                new Video(
+                                        70111470,
+                                        "Die Hard",
+                                        5.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/DieHard150.jpg")
+                                        )),
+                                new Video(
+                                        654356453,
+                                        "Bad Boys",
+                                        4.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys200.jpg"),
+                                                new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys150.jpg")
+                                        ))
+                        )
+                ),
+                new MovieList(
+                        "New Releases",
                         ComposableListExercises.of(
-                            new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/Fracture200.jpg"),
-                            new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/Fracture150.jpg"),
-                            new BoxArt(300, 200, "http://cdn-0.nflximg.com/images/2891/Fracture300.jpg")
-                        ))
+                                new Video(
+                                        65432445,
+                                        "The Chamber",
+                                        4.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber150.jpg")
+                                        )),
+                                new Video(
+                                        675465,
+                                        "Fracture",
+                                        5.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/Fracture200.jpg"),
+                                                new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/Fracture150.jpg"),
+                                                new BoxArt(300, 200, "http://cdn-0.nflximg.com/images/2891/Fracture300.jpg")
+                                        ))
+                        )
                 )
-            )
         );
 
         // Use one or more map, concatAll, and filter calls to create an ComposableList with the following items
@@ -449,8 +414,12 @@ public static ComposableList<JSON> exercise12() {
         //   {"id": 675465,"title": "Fracture","boxart":"http://cdn-0.nflximg.com/images/2891/Fracture150.jpg" },
         // };
 
-        // return movieLists // Complete this expression!
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return movieLists.concatMap(movieList -> {
+            return movieList.videos.concatMap(video -> {
+                ComposableList<BoxArt> boxarts = video.boxarts.filter(boxart -> boxart.height == 200 && boxart.width == 150);
+                return boxarts.map(boxart -> json("id", video.id, "title", video.title, "boxart", boxart.url));
+            });
+        });
     }
 
     /*
@@ -474,24 +443,23 @@ public static BoxArt exercise13() {
                 new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/Fracture150.jpg"),
                 new BoxArt(425, 150, "http://cdn-0.nflximg.com/images/2891/Fracture425.jpg"),
                 new BoxArt(300, 200, "http://cdn-0.nflximg.com/images/2891/Fracture300.jpg")
-            );
-
-            int currentSize = 0;
-            int maxSize = -1;
-            BoxArt largestBoxart = null;
-
-            for (BoxArt boxart: boxarts) {
-                currentSize = boxart.width * boxart.height;
-                if (currentSize > maxSize) {
-                    // ****** INSERT CODE HERE ********
-                    // Assign the largestBoxart to the current boxart, and assign the maxSize to the currentSize.
-                    // ****** INSERT CODE HERE ********
-                }
+        );
+
+        int currentSize = 0;
+        int maxSize = -1;
+        BoxArt largestBoxart = null;
+
+        for (BoxArt boxart : boxarts) {
+            currentSize = boxart.width * boxart.height;
+            if (currentSize > maxSize) {
+                largestBoxart = boxart;
+                maxSize = currentSize;
             }
+        }
 
-        // return largestBoxart;
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return largestBoxart;
     }
+
     /*
     Exercise 14: Implement reduce()
 
@@ -504,7 +472,7 @@ public static BoxArt exercise13() {
     values in the list are exhausted, and then returns a list containing only the last aggregation value. 
     Reduce uses the first value in the list as the initial aggregation value. For example 
     ComposableList.of(1,2,3).reduce((acc,curr) -> acc + curr) is structurally equivalent to ComposableList.of(6).  
-    In the previous example, theBinary function is invoked three times. 
+    In the previous example, theBinary function is invoked three times. (NO, 2 TIMES!)
 
     The first time the binary lambda function is invoked the accumulated value acc is 1 (the first value in the 
     list) and the current value curr is 2 (the second value in the list). The next time the binary lambda function 
@@ -517,9 +485,7 @@ public ComposableList<T> reduce(BiFunction<T, T, T> combiner) {
 
         // If the list is empty, return this
         if (this.size() == 0) {
-            // ************ INSERT CODE HERE **************
-            //  if the list is empty, return this
-            // ********************************************
+            return ComposableListExercises.of();
         } else {
             accumulatedValue = this.get(0);
 
@@ -527,18 +493,12 @@ public ComposableList<T> reduce(BiFunction<T, T, T> combiner) {
             // the previous computation back into the combiner function until
             // we've exhausted the entire list and are left with only one function.
             while (counter < this.size()) {
-                // ****** INSERT CODE HERE ********
-                // Set accumulatedValue to the result of passing accumulatedValue and the list value at the 
-                // counter index to the combiner function.
-                // ****** INSERT CODE HERE ********
-
+                accumulatedValue = combiner.apply(accumulatedValue, this.get(counter));
                 counter++;
             }
 
-            //return ComposableListExercises.of(accumulatedValue);
+            return ComposableListExercises.of(accumulatedValue);
         }
-
-        throw new UnsupportedOperationException("Not implemented yet.");
     }
 
     /*
@@ -568,7 +528,8 @@ public <R> ComposableList<R> reduce(R initialValue, BiFunction<R, T, R> combiner
 
         // If the list is empty, do nothing
         if (this.size() == 0) {
-            return new ComposableListExercises<R>();
+            //return new ComposableListExercises<R>();
+            return ComposableListExercises.of();
         } else {
             counter = 0;
             accumulatedValue = initialValue;
@@ -592,14 +553,14 @@ public <R> ComposableList<R> reduce(R initialValue, BiFunction<R, T, R> combiner
     */
     public static ComposableList<Integer> exercise16() {
         ComposableListExercises<Integer> ratings = ComposableListExercises.of(2, 3, 5, 1, 4);
-        // You should return a list containing only the largest rating. Remember that reduce always
-        // returns a list with one item.
-
-        // complete the expression below
-        //return ratings.reduce
-
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return ratings.reduce((r1, r2) -> {
+            if (r1 > r2) {
+                return r1;
+            }
+            return r2;
+        });
     }
+
     /*
     Exercise 17: Retrieve url of the largest boxart
 
@@ -616,8 +577,15 @@ public static ComposableList<String> exercise17() {
 
         // You should return a list containing only the largest box art. Remember that reduce always
         // returns a list with one item.
-        // return boxarts.reduce  
-        throw new UnsupportedOperationException("Not implemented yet.");
+        // return boxarts.reduce
+        return boxarts.reduce((b1, b2) -> {
+            int size1 = b1.height * b1.width;
+            int size2 = b2.height * b2.width;
+            if (size1 > size2) {
+                return b1;
+            }
+            return b2;
+        }).map(boxart -> boxart.url);
     }
 
     /*
@@ -629,26 +597,26 @@ public static ComposableList<String> exercise17() {
      */
     public static ComposableList<Map<Integer, String>> exercise18() {
         ComposableListExercises<Video> videos = ComposableListExercises.of(
-            new Video(
-                65432445,
-                "The Chamber",
-                5.0
-            ),
-            new Video(
-                675465,
-                "Fracture",
-                4.0
-            ),
-            new Video(
-                70111470,
-                "Die Hard",
-                5.0
-            ),
-            new Video(
-                654356453,
-                "Bad Boys",
-                3.0
-            )
+                new Video(
+                        65432445,
+                        "The Chamber",
+                        5.0
+                ),
+                new Video(
+                        675465,
+                        "Fracture",
+                        4.0
+                ),
+                new Video(
+                        70111470,
+                        "Die Hard",
+                        5.0
+                ),
+                new Video(
+                        654356453,
+                        "Bad Boys",
+                        3.0
+                )
         );
 
         // Expecting this output...
@@ -663,22 +631,34 @@ public static ComposableList<Map<Integer, String>> exercise18() {
 
         // **************** Uncomment code and finish the expression below ******************
         return videos.
-            reduce(
-            // Use an empty map as the initial value instead of the first item in
-            // the list.
-            new HashMap<Integer, String> (),
-            (accumulatedMap, video) -> {
-                // ************ INSERT CODE HERE ************
-                // Remember that you the functions passed to map, filter, concatMap, reduce, and zip can only 
-                // change objects created inside the function. That means that you cannot simply add the video to 
-                // the accumulatedMap. You must make a copy of the accumulated map, and add the video title to that.
-                // Granted, this is very inefficient. However there are libraries that provide immutable maps to make 
-                // copying maps very efficient, allowing this to be a viable approach. For the purpose of this 
-                // exercise simply copy the accumulatedMap into a new map, add the video information to the copy, 
-                // and return the copy.
-                // ************ INSERT CODE HERE ************
-                throw new UnsupportedOperationException("Not implemented yet.");
-            });
+                reduce(
+                        // Use an empty map as the initial value instead of the first item in
+                        // the list.
+                        new HashMap<Integer, String>(),
+                        (accumulatedMap, video) -> {
+                /*
+                Map<Integer, String> map = new HashMap<>();
+                for (Map.Entry<Integer, String> e : accumulatedMap.entrySet()) {
+                    map.put(e.getKey(), e.getValue());
+                }
+                map.put(video.id, video.title);
+                return map;
+
+                 */
+                            accumulatedMap.put(video.id, video.title);
+                            return accumulatedMap;
+                            //"can only change objects created inside the function"--this is not true, if mutating a map is considered changing it.
+                            // In a multithreaded env, it might be a bad idea to mutate it (or it might not), but nothing prevents mutating it.
+                            // ************ INSERT CODE HERE ************
+                            // Remember that you the functions passed to map, filter, concatMap, reduce, and zip can only
+                            // change objects created inside the function. That means that you cannot simply add the video to
+                            // the accumulatedMap. You must make a copy of the accumulated map, and add the video title to that.
+                            // Granted, this is very inefficient. However there are libraries that provide immutable maps to make
+                            // copying maps very efficient, allowing this to be a viable approach. For the purpose of this
+                            // exercise simply copy the accumulatedMap into a new map, add the video information to the copy,
+                            // and return the copy.
+                            // ************ INSERT CODE HERE ************
+                        });
     }
 
     /*
@@ -693,51 +673,51 @@ public static ComposableList<Map<Integer, String>> exercise18() {
     public static ComposableList<JSON> exercise19() {
         ComposableListExercises<MovieList> movieLists = ComposableListExercises.of(
                 new MovieList(
-                    "New Releases",
-                    ComposableListExercises.of(
-                        new Video(
-                            70111470,
-                            "Die Hard",
-                            4.0,
-                            null,
-                            ComposableListExercises.of(
-                                    new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/DieHard150.jpg"),
-                                    new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/DieHard200.jpg")
-                            )),
-                        new Video(
-                            654356453,
-                            "Bad Boys",
-                            5.0,
-                            null,
-                            ComposableListExercises.of(
-                                    new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys200.jpg"),
-                                    new BoxArt(140, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys140.jpg")
-                            ))
-                    )
+                        "New Releases",
+                        ComposableListExercises.of(
+                                new Video(
+                                        70111470,
+                                        "Die Hard",
+                                        4.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/DieHard150.jpg"),
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/DieHard200.jpg")
+                                        )),
+                                new Video(
+                                        654356453,
+                                        "Bad Boys",
+                                        5.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys200.jpg"),
+                                                new BoxArt(140, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys140.jpg")
+                                        ))
+                        )
                 ),
                 new MovieList(
-                    "Thrillers",
-                    ComposableListExercises.of(
-                        new Video(
-                            65432445,
-                            "The Chamber",
-                            3.0,
-                            null,
-                            ComposableListExercises.of(
-                                new BoxArt(130, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber130.jpg"),
-                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber200.jpg")
-                            )),
-                        new Video(
-                            675465,
-                            "Fracture",
-                            4.0,
-                            null,
-                            ComposableListExercises.of(
-                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/Fracture200.jpg"),
-                                new BoxArt(120, 200, "http://cdn-0.nflximg.com/images/2891/Fracture120.jpg"),
-                                new BoxArt(300, 200, "http://cdn-0.nflximg.com/images/2891/Fracture300.jpg")
-                            ))
-                    )
+                        "Thrillers",
+                        ComposableListExercises.of(
+                                new Video(
+                                        65432445,
+                                        "The Chamber",
+                                        3.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(130, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber130.jpg"),
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber200.jpg")
+                                        )),
+                                new Video(
+                                        675465,
+                                        "Fracture",
+                                        4.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/Fracture200.jpg"),
+                                                new BoxArt(120, 200, "http://cdn-0.nflximg.com/images/2891/Fracture120.jpg"),
+                                                new BoxArt(300, 200, "http://cdn-0.nflximg.com/images/2891/Fracture300.jpg")
+                                        ))
+                        )
                 )
         );
 
@@ -756,7 +736,19 @@ public static ComposableList<JSON> exercise19() {
 
             })
          */
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return movieLists.concatMap(movieList -> {
+            return movieList.videos.concatMap(video -> {
+                return video.boxarts
+                        .reduce((b1, b2) -> {
+                            if (b1.width * b1.height < b2.width * b2.height) {
+                                return b1;
+                            }
+                            return b2;
+
+                        })
+                        .map(b -> json("id", video.id, "title", video.title, "boxart", b.url));
+            });
+        });
     }
 
     /*
@@ -793,7 +785,7 @@ public static ComposableList<JSON> exercise20() {
                         5.0
                 )
         );
-        
+
         ComposableListExercises<Bookmark> bookmarks = ComposableListExercises.of(
                 new Bookmark(470, 23432),
                 new Bookmark(453, 234324),
@@ -803,12 +795,11 @@ public static ComposableList<JSON> exercise20() {
         ComposableListExercises<JSON> videoIdAndBookmarkIdPairs = new ComposableListExercises<JSON>();
 
         for (int counter = 0; counter < Math.min(videos.size(), bookmarks.size()); counter++) {
-            // Insert code here to create a {"videoId" : videoId, "bookmarkId" : bookmarkId} JSON 
-            // using json() and add it to the videoIdAndBookmarkIdPairs list.
+            videoIdAndBookmarkIdPairs.add(json("videoId", videos.get(counter).id,
+                    "bookmarkId", bookmarks.get(counter).id));
         }
 
-        // return videoIdAndBookmarkIdPairs;
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return videoIdAndBookmarkIdPairs;
     }
 
     /*
@@ -825,16 +816,14 @@ public static ComposableList<JSON> exercise20() {
         ComposableList.of(4,5,6), 
         (x,y) -> x + y) is structurally equivalent to ComposableList.of(5,7,9)
      */
-    public static <T0,T1,R> ComposableList<R> zip(ComposableList<T0> left, ComposableList<T1> right, BiFunction<T0,T1, R> combinerFunction) {
+    public static <T0, T1, R> ComposableList<R> zip(ComposableList<T0> left, ComposableList<T1> right, BiFunction<T0, T1, R> combinerFunction) {
         ComposableListExercises<R> results = new ComposableListExercises<R>();
 
         for (int counter = 0; counter < Math.min(left.size(), right.size()); counter++) {
-            // Add code here to apply the combinerFunction to the left and right-hand items in the 
-            // respective lists, and add the result to the results List
+            results.add(combinerFunction.apply(left.get(counter), right.get(counter)));
         }
 
-        // return results;
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return results;
     }
 
     /*
@@ -847,36 +836,38 @@ public static <T0,T1,R> ComposableList<R> zip(ComposableList<T0> left, Composabl
     public static ComposableList<JSON> exercise22() {
         ComposableListExercises<Video> videos = ComposableListExercises.of(
                 new Video(
-                70111470,
-                "Die Hard",
-                4.0
-            ),
-            new Video(
-                654356453,
-                "Bad Boys",
-                5.0
-            ),
-            new Video(
-                65432445,
-                "The Chamber",
-                4.0
-            ),
-            new Video(
-                675465,
-                "Fracture",
-                5.0
-            )
+                        70111470,
+                        "Die Hard",
+                        4.0
+                ),
+                new Video(
+                        654356453,
+                        "Bad Boys",
+                        5.0
+                ),
+                new Video(
+                        65432445,
+                        "The Chamber",
+                        4.0
+                ),
+                new Video(
+                        675465,
+                        "Fracture",
+                        5.0
+                )
         );
-        
+
         ComposableListExercises<Bookmark> bookmarks = ComposableListExercises.of(
-            new Bookmark(470, 23432),
-            new Bookmark(453, 234324),
-            new Bookmark(445, 987834)
+                new Bookmark(470, 23432),
+                new Bookmark(453, 234324),
+                new Bookmark(445, 987834)
         );
 
         //... finish this expression
-        // return ComposableListExercises.zip( 
-        throw new UnsupportedOperationException("Not implemented yet.");
+        // return ComposableListExercises.zip(
+        return ComposableListExercises.zip(videos, bookmarks, (video, bookmark) -> {
+            return json("videoId", video.id, "bookmarkId", bookmark.id);
+        });
     }
 
     /*
@@ -890,87 +881,96 @@ with zip(). Return an {id, title, time, url} JSON object for each video.
      */
     public static ComposableList<JSON> exercise23() {
         ComposableListExercises<MovieList> movieLists = ComposableListExercises.of(
-            new MovieList(
-                "New Releases",
-                ComposableListExercises.of(
-                    new Video(
-                        70111470,
-                        "Die Hard",
-                        4.0,
-                        null,
-                        ComposableListExercises.of(
-                            new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/DieHard150.jpg"),
-                            new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/DieHard200.jpg")
-                        ),
-                        ComposableListExercises.of(
-                            new InterestingMoment("End", 213432),
-                            new InterestingMoment("Start", 64534),
-                            new InterestingMoment("Middle", 323133)
-                        )
-                    ),
-                    new Video(
-                        654356453,
-                        "Bad Boys",
-                        5.0,
-                        null,
-                        ComposableListExercises.of(
-                            new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys200.jpg"),
-                            new BoxArt(140, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys140.jpg")
-                        ),
-                        ComposableListExercises.of(
-                            new InterestingMoment("End", 54654754),
-                            new InterestingMoment("Middle", 6575665)
-                        )
-                    )
-                )
-            ),
-            new MovieList(
-                "Instant Queue",
-                ComposableListExercises.of(
-                    new Video(
-                        65432445,
-                        "The Chamber",
-                        4.0,
-                        null,
-                        ComposableListExercises.of(
-                            new BoxArt(130, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber130.jpg"),
-                            new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber200.jpg")
-                        ),
+                new MovieList(
+                        "New Releases",
                         ComposableListExercises.of(
-                            new InterestingMoment("End", 132423),
-                            new InterestingMoment("Start", 54637425),
-                            new InterestingMoment("Middle", 3452343)
+                                new Video(
+                                        70111470,
+                                        "Die Hard",
+                                        4.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(150, 200, "http://cdn-0.nflximg.com/images/2891/DieHard150.jpg"),
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/DieHard200.jpg")
+                                        ),
+                                        ComposableListExercises.of(
+                                                new InterestingMoment("End", 213432),
+                                                new InterestingMoment("Start", 64534),
+                                                new InterestingMoment("Middle", 323133)
+                                        )
+                                ),
+                                new Video(
+                                        654356453,
+                                        "Bad Boys",
+                                        5.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys200.jpg"),
+                                                new BoxArt(140, 200, "http://cdn-0.nflximg.com/images/2891/BadBoys140.jpg")
+                                        ),
+                                        ComposableListExercises.of(
+                                                new InterestingMoment("End", 54654754),
+                                                new InterestingMoment("Middle", 6575665)
+                                        )
+                                )
                         )
-                    ),
-                    new Video(
-                        675465,
-                        "Fracture",
-                        5.0,
-                        null,
-                        ComposableListExercises.of(
-                            new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/Fracture200.jpg"),
-                            new BoxArt(120, 200, "http://cdn-0.nflximg.com/images/2891/Fracture120.jpg"),
-                            new BoxArt(300, 200, "http://cdn-0.nflximg.com/images/2891/Fracture300.jpg")
-                        ),
+                ),
+                new MovieList(
+                        "Instant Queue",
                         ComposableListExercises.of(
-                            new InterestingMoment("End", 45632456),
-                            new InterestingMoment("Start", 234534),
-                            new InterestingMoment("Middle", 3453434)
+                                new Video(
+                                        65432445,
+                                        "The Chamber",
+                                        4.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(130, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber130.jpg"),
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/TheChamber200.jpg")
+                                        ),
+                                        ComposableListExercises.of(
+                                                new InterestingMoment("End", 132423),
+                                                new InterestingMoment("Start", 54637425),
+                                                new InterestingMoment("Middle", 3452343)
+                                        )
+                                ),
+                                new Video(
+                                        675465,
+                                        "Fracture",
+                                        5.0,
+                                        null,
+                                        ComposableListExercises.of(
+                                                new BoxArt(200, 200, "http://cdn-0.nflximg.com/images/2891/Fracture200.jpg"),
+                                                new BoxArt(120, 200, "http://cdn-0.nflximg.com/images/2891/Fracture120.jpg"),
+                                                new BoxArt(300, 200, "http://cdn-0.nflximg.com/images/2891/Fracture300.jpg")
+                                        ),
+                                        ComposableListExercises.of(
+                                                new InterestingMoment("End", 45632456),
+                                                new InterestingMoment("Start", 234534),
+                                                new InterestingMoment("Middle", 3453434)
+                                        )
+                                )
                         )
-                    )
                 )
-            )
         );
 
-        //------------ COMPLETE THIS EXPRESSION --------------
-        /*
-        return movieLists.
-            concatMap(movieList -> {
-
-            });
-        */
-
-        throw new UnsupportedOperationException("Not implemented yet.");
+        return movieLists.concatMap(movieList -> {
+           return movieList.videos.concatMap(video -> {
+               ComposableList<BoxArt> boxarts = video.boxarts.reduce((b1, b2) -> {
+                   if (b1.height * b1.width < b2.height * b2.width) {
+                       return b1;
+                   }
+                   return b2;
+               });
+               ComposableList<InterestingMoment> moments = video.interestingMoments.filter(im -> {
+                   return im.type.equals("Middle");
+               });
+               return boxarts.concatMap(ba -> {
+                   return moments.map(m -> {
+                      return json("id", video.id, "title", video.title, "time", m.time, "url", ba.url);
+                   });
+               });
+           });
+        });
     }
 
 
@@ -994,7 +994,7 @@ public static <T> ComposableListExercises<T> of(T... args) {
         }
         return results;
     }
-    
+
     public static <T> ComposableListSolutions<T> of(List<T> list) {
         ComposableListSolutions<T> results = new ComposableListSolutions<T>();
         for (T value : list) {
diff --git a/src/main/java/learnrxjava/ObservableExercises.java b/src/main/java/learnrxjava/ObservableExercises.java
index a7c5d00..835186c 100644
--- a/src/main/java/learnrxjava/ObservableExercises.java
+++ b/src/main/java/learnrxjava/ObservableExercises.java
@@ -1,7 +1,6 @@
 package learnrxjava;
 
-import learnrxjava.types.JSON;
-import learnrxjava.types.Movies;
+import learnrxjava.types.*;
 import rx.Observable;
 
 public class ObservableExercises {
@@ -12,7 +11,7 @@ public class ObservableExercises {
      * @return "Hello World!"
      */
     public Observable<String> exerciseHello() {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return Observable.just("Hello World!");
     }
 
     /**
@@ -21,7 +20,10 @@ public Observable<String> exerciseHello() {
      * @param "Hello Name!"
      */
     public Observable<String> exerciseMap(Observable<String> hello) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return Observable.just("Hello World!")
+                .map(s -> {
+                   return s.replace("World!", "Dan");
+                });
     }
 
     /**
@@ -32,17 +34,24 @@ public Observable<String> exerciseMap(Observable<String> hello) {
      * 6-Even
      */
     public Observable<String> exerciseFilterMap(Observable<Integer> nums) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return Observable.just(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+                .filter(i -> {
+                    return i % 2 == 0;
+                }).map(i -> "" + i + "-Even");
     }
 
     /**
      * Flatten out all video in the stream of Movies into a stream of videoIDs
      * 
-     * @param movieLists
+     * @param movies
      * @return Observable of Integers of Movies.videos.id
      */
     public Observable<Integer> exerciseConcatMap(Observable<Movies> movies) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return movies.concatMap(m -> {
+            return m.videos.map(movie -> {
+               return movie.id;
+            });
+        });
     }
 
     /**
@@ -55,11 +64,13 @@ public Observable<Integer> exerciseConcatMap(Observable<Movies> movies) {
      * 
      * We'll see more about this later when we add concurrency.
      * 
-     * @param movieLists
+     * @param movies
      * @return Observable of Integers of Movies.videos.id
      */
     public Observable<Integer> exerciseFlatMap(Observable<Movies> movies) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return movies.flatMap(movieList -> {
+            return movieList.videos.map(movie -> movie.id);
+        });
     }
 
     /**
@@ -68,7 +79,12 @@ public Observable<Integer> exerciseFlatMap(Observable<Movies> movies) {
      * Use reduce to select the maximum value in a list of numbers.
      */
     public Observable<Integer> exerciseReduce(Observable<Integer> nums) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return nums.reduce((i1, i2) -> {
+            if (i1 > i2) {
+                return i1;
+            }
+            return i2;
+        });
     }
 
     /**
@@ -83,7 +99,18 @@ public Observable<Integer> exerciseReduce(Observable<Integer> nums) {
      * See Exercise 19 of ComposableListExercises
      */
     public Observable<JSON> exerciseMovie(Observable<Movies> movies) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return movies.flatMap(movieList -> {
+            return movieList.videos.flatMap(movie -> {
+                return movie.boxarts.reduce((b1, b2) -> {
+                    if (b1.width * b1.height < b2.width * b2.height) {
+                        return b1;
+                    }
+                    return b2;
+                }).map(b -> {
+                   return json("id", movie.id, "title", movie.title, "boxart", b.url);
+                });
+            });
+        });
     }
 
     /**
@@ -94,7 +121,9 @@ public Observable<JSON> exerciseMovie(Observable<Movies> movies) {
      * output -> "one fish", "two fish", "red fish", "blue fish"
      */
     public Observable<String> exerciseZip(Observable<String> a, Observable<String> b) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return Observable.zip(a, b, (a1, b1) -> {
+            return a1 + " " + b1;
+        });
     }
 
     /**
@@ -102,7 +131,9 @@ public Observable<String> exerciseZip(Observable<String> a, Observable<String> b
      * and replace it with "default-value".
      */
     public Observable<String> handleError(Observable<String> data) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return data.map(d -> {
+            return d;
+        }).onErrorReturn(t -> "default-value");
     }
 
     /**
@@ -110,7 +141,9 @@ public Observable<String> handleError(Observable<String> data) {
      * with retry capability.
      */
     public Observable<String> retry(Observable<String> data) {
-        return Observable.error(new RuntimeException("Not Implemented"));
+        return data.map(d -> {
+            return d;
+        }).retry(5);
     }
 
     // This function can be used to build JSON objects within an expression