15 20 is a very popular hand guessing game.
It's as easy as rock-paper-scissors, but more exciting.

How to play:

On your turn, select your call (0, 5, 10, 15, or 20). Swipe the screen with 2 fingers simultaneously to show your hands. Upwards is palm, downwards is fist. Palm is 5, fist is 0. Your opponent needs to respond immediately (less than a second) by swiping the screen with 2 fingers to avoid your call. The total count of palms and fists must not equal to your call. Your opponent can choose not to swipe if the count is already different from your call. If your opponent successfully avoided your call, it's his/her turn to call and all scores are reset to 0. If your opponent failed, you score a point and get to call again. You win a round by scoring 2 consecutive points. Whoever wins a round, all hands are reset to fists. The first to win 10 rounds wins the match.

source : itunes.apple.com