Description

Adds Return on Investment (ROI) to the math category.

ROI is a foundational financial formula that measures how profitable an investment was relative to what you put in — it shows up in everything from business analysis to personal finance:

ROI = (Gain from Investment - Cost of Investment) / Cost of Investment × 100

What's included

• src/algorithms/math/return-on-investment/ReturnOnInvestment.js — clean implementation, throws on invalid cost
• src/algorithms/math/return-on-investment/__test__/ReturnOnInvestment.test.js — 6 test cases covering all branches
• src/algorithms/math/return-on-investment/README.md — explanation, formula table, and reference

Checklist

• Follows the repo's directory and file naming conventions
• Jest tests for positive/zero/negative ROI and error cases
• README with formula, examples table, and Investopedia reference

Reference: https://www.investopedia.com/terms/r/returnoninvestment.asp