(1) one-step calculation: 2+9, 17-6, 9-4, 20-9, 6+6, 15- 10.

(2) two-step calculation: 3+8-5, 7-5+9, 9+8-5, 18-7-5, 4-1+1,4+5+6.

to be divided three classes

(1) Addition formula: 2+9, 6+6, 4+5+6.

(2) Subtraction formula: 17-6, 9-4, 20-9, 18-7-5,10.

(3) Addition and subtraction formulas: 3+8-5, 7-5+9, 9+8-5, 4-1+1.

Extended data 1, elementary arithmetic order: when operating at the same level, it is calculated from left to right in turn; Two-stage operation, first multiply and then divide, then add and subtract. When there are brackets, count the inside of brackets first, and then count the outside of brackets; When there are multiple brackets, count the brackets first, then the brackets inside, then the braces inside, and finally the brackets outside. If there is a power, calculate the power first.

2. Multiplication is a simple operation of addition and division is a simple operation of subtraction. Subtraction and addition are reciprocal operations, and division and multiplication are reciprocal operations. When several addends are added, the positions of addends can be exchanged at will; Or add a few addends first and then add them with other addends, and the sum remains the same. Subtracting the sum of two numbers from a number is equivalent to subtracting each addend in the sum from this number in turn.