Add return on investment to math algorithms

Author: InukaWijerathna

src/algorithms/math/return-on-investment/README.md

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+# Return on Investment (ROI)
+
+Return on Investment (ROI) is a financial metric that measures the profitability
+of an investment relative to its cost. It is expressed as a percentage.
+
+## Formula
+
+```
+ROI = (Gain from Investment - Cost of Investment) / Cost of Investment × 100
+```
+
+## Examples
+
+| Gain   | Cost  | ROI     |
+|--------|-------|---------|
+| $1000  | $500  | 100%    |
+| $500   | $500  | 0%      |
+| $200   | $500  | -60%    |
+| $0     | $500  | -100%   |
+
+## References
+
+- [Investopedia - Return on Investment](https://www.investopedia.com/terms/r/returnoninvestment.asp)

src/algorithms/math/return-on-investment/ReturnOnInvestment.js

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+/**
+ * @param {number} gainFromInvestment
+ * @param {number} costOfInvestment
+ * @return {number}
+ */
+export default function returnOnInvestment(gainFromInvestment, costOfInvestment) {
+  if (costOfInvestment <= 0) {
+    throw new Error('costOfInvestment must be greater than 0');
+  }
+  return ((gainFromInvestment - costOfInvestment) / costOfInvestment) * 100;
+}

src/algorithms/math/return-on-investment/__test__/ReturnOnInvestment.test.js

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+import returnOnInvestment from '../ReturnOnInvestment';
+
+describe('returnOnInvestment', () => {
+  it('should calculate positive ROI', () => {
+    expect(returnOnInvestment(1000, 500)).toBe(100);
+  });
+
+  it('should return zero ROI when gain equals cost', () => {
+    expect(returnOnInvestment(500, 500)).toBe(0);
+  });
+
+  it('should calculate negative ROI', () => {
+    expect(returnOnInvestment(200, 500)).toBe(-60);
+  });
+
+  it('should return -100 for total loss', () => {
+    expect(returnOnInvestment(0, 500)).toBe(-100);
+  });
+
+  it('should throw on zero cost', () => {
+    expect(() => returnOnInvestment(1000, 0)).toThrow();
+  });
+
+  it('should throw on negative cost', () => {
+    expect(() => returnOnInvestment(1000, -100)).toThrow();
+  });
+});