The mental arithmetic formula within 20 is as follows: 1, carry addition. Formula: "Add nine and subtract one, add eight and subtract two, add seven and subtract three, add six and subtract four, and add five and subtract five." How to use the formula? Take "plus nine MINUS one" as an example. "Add nine and subtract one" refers to the addition of a number and 9, and the number minus 1 is the unit of their sum. For example, 8+9=(_), take _8 minus 1 to get 7, use 7 as the unit of sum, that is, 8+9= 17, _5+9=(_), take 5 minus 1 to get 4, and use 4 as the unit of sum, that is, 5+9. Formula: "minus nine plus one, minus eight plus two, minus seven plus three, minus six plus four, minus five plus five." How to use the formula? Take "minus nine plus one" as an example. "Minus nine plus one" means a number minus nine, and the result of adding 1 to this number is their difference. Example 1: 17-9 = (_) Take the 7th place of 17 and add 1, and the result is 8, that is, 17-9 = 8, 13-9 = Example 2: 17-2 = (_) Distinguish between numbers and decimals. Let's see if the single digits can come down first. If 7-2 is enough, subtract 10, remember 7 in your mind, and then count down to 6, 5 to get 5. Then, the decimal number 1 remains unchanged, and you get 15.