The humble calculator has never been so fun.
Players in this game are presented with a goal and a limited number of moves in which to reach it. But you don't have access to the full range of mathematical operations. You're given just a few mathematical possibilities (i.e., +2, -4, x5) and also some creative tools (such as using the delete key to turn 200 into 20) to hit your goal.
In addition to being educational and fun, Calculator: The Game also manages to inject a little personality into the calculator character, which is an added bonus.
The game is ad-supported. If you get stumped, you can ask for hints — but if you need a lot of help, you have to pay for them. It is perfectly possible to solve the puzzles without paying.