1。 Addition of two numbers: 2+3=5, 4+5=9.

2。 Addition of 3 numbers: 4+2+3=9, 2+2+3 = 7, 1+ 1+7 = 9.

3。 2 number subtraction: 8- 1 = 7, 7-2 = 5, 10-3 = 7.

4。 3 number subtraction: 9- 1-3=5, 10- 1-2=7,

One point: the result is (9) there are _ 3 _; Yes (7) yes _ 4 _; Have (5) have _3_. According to (algorithm), there are two categories: _ addition _ _, _ subtraction _ _.

According to the number of operations, it can be divided into (two) categories: _2 number operation _ _, _3 number operation _ _.