using System;

using System.Collections.Generic;

public class Program

{

    public class Item 

    {

        public decimal UnitPrice {get;set;}

        public int Quantity {get;set;}

    }

    public static void Main()

    {

        var holdingItems = new List<Item>() {

            new Item { UnitPrice = 150, Quantity = 2 },

            new Item { UnitPrice = 50, Quantity = 3 },

            new Item { UnitPrice = 25, Quantity = 6 },

        };

​

        var taxTotal = (decimal)42;

        var remainingTax = (decimal)taxTotal; 

        var orderSubtotal = (decimal)600; 

        foreach (var item in holdingItems)

        {

            var extendedPrice = item.UnitPrice * item.Quantity;

            var allocatedTax = taxTotal * (extendedPrice / orderSubtotal);

            item.UnitPrice += allocatedTax / item.Quantity;

            remainingTax -= allocatedTax;

        }

        Console.WriteLine(remainingTax);

    }

}