Primary school estimation generally follows the principle of rounding. Examples are as follows: 150+3 17. The estimation process is to regard 150 as 200 and the other 3 17 as 300. It can be calculated that the estimated result of 150+3 17 is 500.

For example: 53, 57, 5 1, 59, these numbers are all close to 55, some are greater than 55, and some are less than 55. Take an intermediate number 55, and directly use 55×4 to roughly calculate the result of adding these four numbers.