Just consulted the child's math teacher and said that there are two ways: one is to estimate to the whole hundred according to the principle of rounding, and then work out the answer; 2. Round the answer to ten digits and work out the answer. However, it should be noted that the methods used in a topic should be consistent, and the first number should not be estimated by the whole hundred, and the second number should be estimated by the whole ten. If you calculate first and then estimate, it is not an estimate, but a divisor.

For example: 38 1+252=380+250=630.

603+ 199=600+200=800