using System;

​

public class Program

{

    public static void Main()

    {

        //Change these four variables to change the prorated price. Year,Month,Day for the DateTimes.

        decimal oldClubPrice = 14.99m;

        decimal newClubPrice = 19.99m;

        DateTime upgradeDate = new DateTime(2020, 3, 18);

        DateTime nextBillingDate = new DateTime(2020, 5, 16);

        var billableDays = nextBillingDate.Subtract(upgradeDate).Days;

        decimal diff = newClubPrice - oldClubPrice;

        decimal proratedPrice = diff;

        if (billableDays > 30)

        {

            billableDays = billableDays - 30;

            proratedPrice = Math.Round(Convert.ToDecimal((diff / 30.4m) * billableDays), 2);

        }

​

        Console.WriteLine("${0}",proratedPrice);

    }

}