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agentd task 4027a3aa01be47e9b677e7c687e4b183

src/Prices/PriceCalculator.cs

@@ -16,6 +16,57 @@ public static class PriceCalculator
             throw new ArgumentOutOfRangeException(nameof(discountPercent), "A discount is between 0 and 100 percent.");
         }
 
-        return price - discountPercent;
+        return price * (1m - discountPercent / 100m);
+    }
+
+    /// <summary>Rounds an amount to the nearest 0.05, rounding halves away from zero.</summary>
+    public static decimal RoundToFiveRappen(decimal amount)
+    {
+        return Math.Round(amount * 20m, MidpointRounding.AwayFromZero) / 20m;
+    }
+
+    /// <summary>Returns the quantity discount percent for a quantity: 0, 5 or 10.</summary>
+    public static int GetQuantityDiscountPercent(int quantity)
+    {
+        if (quantity < 1)
+        {
+            throw new ArgumentOutOfRangeException(nameof(quantity), "A quantity must be at least 1.");
+        }
+
+        return quantity switch
+        {
+            >= 50 => 10,
+            >= 10 => 5,
+            _ => 0,
+        };
+    }
+
+    /// <summary>
+    /// Calculates the total: percentage discount on the unit price first, then the quantity
+    /// discount on the total, then rounds the end amount to the nearest 0.05.
+    /// </summary>
+    public static decimal CalculateTotal(decimal unitPrice, int quantity, decimal discountPercent)
+    {
+        if (unitPrice < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(unitPrice), "A price cannot be negative.");
+        }
+
+        if (quantity < 1)
+        {
+            throw new ArgumentOutOfRangeException(nameof(quantity), "A quantity must be at least 1.");
+        }
+
+        if (discountPercent is < 0 or > 100)
+        {
+            throw new ArgumentOutOfRangeException(nameof(discountPercent), "A discount is between 0 and 100 percent.");
+        }
+
+        var discountedUnitPrice = ApplyDiscount(unitPrice, discountPercent);
+        var totalBeforeQuantityDiscount = discountedUnitPrice * quantity;
+        var quantityDiscountPercent = GetQuantityDiscountPercent(quantity);
+        var totalBeforeRounding = totalBeforeQuantityDiscount * (1m - quantityDiscountPercent / 100m);
+
+        return RoundToFiveRappen(totalBeforeRounding);
     }
 }