n-back

＜ USAGE ＞ 1. Select level (1-10). 2. Memorize the answer of easy addition for the level times. 3. Continue memorizing and tap the last digit of the answer of n times before (NOT current answer). You can submit your score to Google Play Game If you don't know what is N-Back Task, please see below. http://en.wikipedia.org/wiki/N-back