using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(String.Format("{0}",100.00));

        Console.WriteLine(String.Format("{0}",0.5));

        DateTime startOfWeeks=DateTime.Now;

         while (startOfWeeks.DayOfWeek != DayOfWeek.Monday) startOfWeeks = startOfWeeks.AddDays(-1);

        Console.WriteLine(startOfWeeks.ToString());

        //DateTime dt = new Program().StartOfWeek(DateTime.Now,DayOfWeek.Friday);

        //Console.WriteLine(dt.ToString());

    }

    public DateTime StartOfWeek(DateTime dt, DayOfWeek startOfWeek)

    {

        int diff = dt.DayOfWeek - startOfWeek;

        if (diff < 0)

        {

            diff += 7;

        }

        return dt.AddDays(-1 * diff).Date;

    }

}

​