book-store: use book price const in calculation

ferhatelmas · ferhatelmas · commit 31402db61736 · 2017-10-19T22:31:50.000+02:00
diff --git a/exercises/book-store/example.py b/exercises/book-store/example.py
@@ -5,7 +5,7 @@ def _group_price(size):
     discounts = [0, .05, .1, .2, .25]
     if not (0 < size <= 5):
         raise ValueError('size must be in 1..' + len(discounts))
-    return 8 * size * (1 - discounts[size - 1])
+    return BOOK_PRICE * size * (1 - discounts[size - 1])
 
 
 def calculate_total(books, price_so_far=0.):
