1, rule 1: Without brackets, if there is only addition and subtraction, or only multiplication and division, it is calculated from left to right. In fact, we divide addition and subtraction into one level and multiplication and division into one level.

2. Rule 2: Without brackets, if there are multiplication, division and addition and subtraction, multiply and divide first, and then add and subtract. What we should pay attention to here is priority, and multiplication and division take precedence over addition and subtraction.

Rule 3: If there are brackets in the formula, calculate the numbers in brackets first.