I'm working on a simple calculator to add, subtract, multiply, divide, and mod(%). It is written in a GUI format with C#. I am currently stuck on a bug that doesn't calculate digits over 9. Here is the progam so far, it will work with digits like 20 + 20, 20 * 20, just play with it. Open the .sln with Visual Studio 2008 or Visual C# express edition(recommended), available from Microsoft here. or even simpler just run the calculator.exe in bin|release|calculator.exe. Warning: This program has many bugs. I am still working on it. But the basic functionality is complete.