Operation order: from left to right.

The reason for this is that in mathematical calculation, because addition and subtraction belong to the same level of operation, it is usually calculated from left to right in turn. In the calculation, if there are brackets, you must first calculate the numbers in brackets. If there is a simple calculation, it can be done by additive commutative law, the law of addition and the nature of subtraction.

However, if there is a two-level operation, the second-level operation should be performed first, and then the first-level operation should be performed from left to right. The most common ones are addition, subtraction, multiplication and division.

Characteristics of addition and subtraction mixed operation:

When writing vertically, you can calculate the addition and subtraction mixed operation step by step. The method is the same as adding (subtracting) two numbers. The same numbers should be aligned and counted from one number. You can also write in a simple way, in a vertical row. First, complete the calculation of the first step, and then add (subtract) the second number with the result of the first step.

In daily life, we don't need accurate calculation, we just need to work out a rough result. In this case, we need to estimate. When estimating, estimate this number to the nearest integer ten and then calculate.

When writing vertically, you can calculate the addition and subtraction mixed operation step by step. The method is the same as adding (subtracting) two numbers. The same numbers should be aligned and counted from one number. You can also write in a simple way, in a vertical row. First, complete the calculation of the first step, and then add (subtract) the second number with the result of the first step.