Add project files.

BrokerageLib/CommissionCalculator.cs

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+using System;
+
+namespace BrokerageLib {
+
+	public class CommissionCalculator {
+
+		
+		public decimal DetermineVariableRate(int unitsSold, decimal unitPrice) {
+
+			// Sales representative gets top commission rate
+			// if they sell over the sales threshold amount
+			// or if they sell more than the max unit threshold
+			if (unitsSold < 0)
+			{
+				throw new ArgumentOutOfRangeException("UnitsSold cannot be less than zero.");
+			}
+
+			if (unitPrice < 0)
+			{
+				throw new ArgumentOutOfRangeException("unitPrice cannot be less than zero.");
+			}
+
+			decimal grossSale = unitsSold * unitPrice;
+			if (grossSale > Constants.CommissionThreshold.SalesAmount || unitsSold > Constants.CommissionThreshold.UnitAmount)
+			{
+				return grossSale * Constants.CommissionRate.Top;
+			}
+			else
+			{
+				
+				return grossSale * Constants.CommissionRate.Standard;
+			}
+		}
+	}
+}