implement IRateFetcher interface

Credit/CreditSimulator.cs

-namespace Credit;
-
-public class MortgageSimulator
-{
-    public static Amount ComputeMonthlyPayment(Amount borrowedAmount, int durationInMonth, decimal rate)
-    {
-        if (borrowedAmount.Value * (int) Math.Pow(10, borrowedAmount.Currency.Precision) < 5000000) {
-            throw new ArgumentOutOfRangeException(nameof(borrowedAmount), "Can't borrow so little capital.");
-        }
-
-        double numerator = (double)(borrowedAmount * rate / 12);
-        double denominator = 1 - Math.Pow((double)(1  + rate / 12), - durationInMonth);
-        double value = numerator / denominator;
-        
-        return new Amount(value, borrowedAmount.Currency);
-    }
-}
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