The main method of addition and subtraction on the abacus is understanding and using the 5 complement pairs of 10, i.e. pairs of numbers that add to 10. There are only 5 complement pairs in our base 10 numbering system: 9 and 1, 8 and 2, 7 and 3, 6 and 4, and 5 and 5. The faster students can recall the 10 pair complements, the faster they can add and subtract on the soroban. So students should practice recalling all of the 10 pair complements with the goal of reaching instant recall. Here a couple good 10 pair complement activities to practice recalling the pairs as fast as possible.