Add ReturnOnInvestment to math

src/main/java/com/williamfiset/algorithms/math/ReturnOnInvestment.java

@@ -0,0 +1,29 @@
+/**
+ * Calculates Return on Investment (ROI) as a percentage.
+ *
+ * <p>ROI measures the profitability of an investment relative to its cost.
+ *
+ * <p>Formula: ROI = (Gain - Cost) / Cost × 100
+ *
+ * <p>Reference: https://www.investopedia.com/terms/r/returnoninvestment.asp
+ *
+ * @author InukaWijerathna
+ */
+package com.williamfiset.algorithms.math;
+
+public class ReturnOnInvestment {
+
+  public static double roi(double gainFromInvestment, double costOfInvestment) {
+    if (costOfInvestment <= 0) {
+      throw new IllegalArgumentException("costOfInvestment must be greater than 0");
+    }
+    return (gainFromInvestment - costOfInvestment) / costOfInvestment * 100.0;
+  }
+
+  public static void main(String[] args) {
+    System.out.println(roi(1000, 500));  // 100.0
+    System.out.println(roi(500, 500));   // 0.0
+    System.out.println(roi(200, 500));   // -60.0
+    System.out.println(roi(0, 500));     // -100.0
+  }
+}