This is the same question as you asked yesterday:
Decrease value of textbox within another textbox[
^]
The answer hasn't changed, nor will it - ZurdoDev's answer is the same as the one I gave you then.
We aren't going to write all the code for you - you wouldn't learn anything if we did. SO follow the C# links I gave you, read the answers ZurdoDev and I wrote, and start working out exactly what to type. We aren't going to give you something you can hand in as your own work...