Add Up to 21

Poker Blackjack is a popular game related to adding numbers to 21. This app adopts this idea to train your brain to add number fast without paper and pen.
You are challenged to do this as fast as you can within 3 minutes: pick cards from the 7 given number-cards so that the sum of them is 21. Each number-card is a poker card with a value in the range of 1 to 13.
The more you practice, the faster you will be able to pick the right numbers for the sum of 21. Doing this can truly sharpen your skills and performance to computing in brain without using pen and paper.
Your best performance is saved, along with the date and time you created it.
This app is for pre-school, 1st grade, 2nd grade students and senior citizens who want to keep their brain sharp.