Adding 1 is not enough, and adding 9 is not enough.

The reduction of 1 is not enough, and the double increase is 9.

Adding 2 below is not enough, adding 8 below is enough.

Less than two is not enough, more than eight plus.

Adding 3 below is not enough, adding 7 below is enough.

Less than 3 is not enough, more than 7.

Adding 4 below is not enough, adding 6 below is enough.

Less than 4 is not enough, but more than 6.

Fast mental arithmetic is different from abacus mental arithmetic and palm mental arithmetic. ?

Mainly through certain rules in textbooks, children are trained to operate quickly in addition, subtraction, multiplication and division.

Quick mental arithmetic is not abacus mental arithmetic/palm mental arithmetic. You can quickly write the correct answer by reading the questions without any auxiliary tools (such as abacus/fingers). Fast heart is a fast calculation method that is consistent with the primary school mathematics calculation method, but simpler than the primary school textbook. Easy to learn, can really reduce the burden on children.

Baidu encyclopedia-quick mental arithmetic