I realize you may not want to share it, but if you have some sort of resource you could point me at I'd appreciate it, since I also like doing basic arithmetic as a mental game. I view it as similar to the type of casual timekilling that one would do with sudoku or minesweeper, but at least semi-productive in that I'm building mental arithmetic skills. If you feel inclined to share the source so I could run my own instance, I can be reached at my username + google's mail service.