Mental arithmetic, the handle is a five-bead abacus. Children's kindergarten teaches ten on the left and one on the right, and they have to recite formulas. Only the left thumb is 50, and the right thumb stands for 5, except that each finger on the left thumb is 10 and the right thumb is 1. If kindergarten students can't recite formulas, they should use hand index, such as 4+2=6. After going, they have to stretch out four thumbs first, and then take back the other four thumbs once, which is equivalent to counting abacus. 2 is to count two, starting with the thumb, counting from the first thumb, counting the other four fingers, and adding one second. Children will carry a pistol form, which is 6, so an answer is equal to 6, and that's how children calculate it.

When the thumb counts twice, use the left hand to index. The second round is the same as the abacus, except that the child does not recite the formula and only remembers how many times the thumb counts. 6+7= 13, and 6 bends its thumb once, with six at the beginning. The children remember that it was a pistol gesture and continue to count down. At this time, write 1 in your left hand, extend 1 in your left hand, and make a fist after counting to your thumb with your right hand, and take it all back. Count a finger from the fifth, so 10 on the left and 3 on the right is 13, which is what kindergarten teaches children to add and subtract within 99.