1, and: known part, and.

For example, Xiaoming does 8 math problems in the morning and 3 math problems in the afternoon. How many math problems does Xiaoming do every day?

2. Find the surplus: after knowing the total amount and used parts, find the surplus.

For example: Mom bought 10 apples, and Xiaoming ate four. How many apples are left?

3. Seek comparison: Given two parts, seek more or less.

For example, Xiaoming is 9 years old and Xiaoli is 7 years old. How old is Xiaoming? (Xiaoli is several years younger than Xiaoming)