MathTap

MathTap is an revolutionary, but simple game. It easily is based on subtraction of numbers.
You start with a score of 50 and you have to tap as many tiles, which are placed in a 4 by 4 grid until you get to zero. The tiles could either add the value, displayed on the tile or subtract it, depending on their color.
If you get to zero, the next levels begins, where you have to subtract a greater amount of score.
Sounds simple, but boring. To make it a little more spicy, there is a limit on how many taps you can do. After you reached the end of a level, you get a new bunch of taps.
The game ends when you have no taps left or your score goes negative. Then all your subtracted score is add to get your endscore, which you can use to compete with you friends.
Enough of the theory, download MathTap and dive into the world of subtraction.