There is a game called flash mental arithmetic as free children games(Flash anzan).
This is a simple game where the numbers that appear on the screen are added by mental arithmetic, but as the number of digits increases to 2 digits and 3 digits, knowledge of the abacus and mental arithmetic are required. You can learn everything at online-soroban.com.
There are 5 questions for each item, and there is also a ranking board, so you can check how much your grades are. You can play for free, so please play with everyone.
The current world record is 1.66 seconds within 15 additions of 3 digits.（2015）(Using abacus skill and imagination to count by head)