XDA-Developers member cubewalker has developed a game called Math Scramble that I think is a great concept. As you can see from the image above, it gives you the answer to a math equation, and then challenges you to quickly come up with the equation that gives that solution. More points and bonus time is given for longer equations. A leader board is under development.
Hit the source link for more info and the download link, and be sure to support the developer if you like his work.