using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        Console.WriteLine($"125.00m --> {125.00m.RoundDownToNearest(0.25m).ToTrimmedString()}");

        Console.WriteLine($"125.25m --> {125.25m.RoundDownToNearest(0.25m).ToTrimmedString()}");

        Console.WriteLine($"125.33m --> {125.33m.RoundDownToNearest(0.25m).ToTrimmedString()}");

        Console.WriteLine($"125.50m --> {125.50m.RoundDownToNearest(0.25m).ToTrimmedString()}");

        Console.WriteLine($"125.66m --> {125.66m.RoundDownToNearest(0.25m).ToTrimmedString()}");

        Console.WriteLine($"125.75m --> {125.75m.RoundDownToNearest(0.25m).ToTrimmedString()}");

        Console.WriteLine($"125.99m --> {125.99m.RoundDownToNearest(0.25m).ToTrimmedString()}");

    }

​

}

​

public static class Exts

{

    public static decimal RoundDownToNearest(this decimal passednumber, decimal roundto)

    {

        if (roundto == 0)

        {

            return passednumber;

        }

​

        return Math.Floor(passednumber / roundto) * roundto;

    }

    public static string ToTrimmedString(this decimal amount)

    {

        var output = amount.ToString(CultureInfo.InvariantCulture);

​

        return output.Contains(".") ?

            output.TrimEnd('0').TrimEnd('.') :

            output;

    }

}