diff --git a/Maths/ReturnOnInvestment.php b/Maths/ReturnOnInvestment.php
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+++ b/Maths/ReturnOnInvestment.php
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+<?php
+
+/**
+ * Calculates Return on Investment (ROI) as a percentage.
+ * ROI measures the profitability of an investment relative to its cost.
+ *
+ * Formula: ROI = (Gain - Cost) / Cost * 100
+ *
+ * @see https://www.investopedia.com/terms/r/returnoninvestment.asp
+ *
+ * @param float $gainFromInvestment total value gained from the investment
+ * @param float $costOfInvestment   total cost of the investment
+ * @return float ROI as a percentage
+ * @throws \InvalidArgumentException if costOfInvestment is not positive
+ */
+function returnOnInvestment(float $gainFromInvestment, float $costOfInvestment): float
+{
+    if ($costOfInvestment <= 0) {
+        throw new \InvalidArgumentException('costOfInvestment must be greater than 0');
+    }
+    return ($gainFromInvestment - $costOfInvestment) / $costOfInvestment * 100;
+}