Among the four operations, addition and subtraction are called primary operations, and multiplication and division are called secondary operations. Expressions that contain two or more operations are usually called mixed operations.
Peer operation In an expression, if it only contains peer operation, it should be operated from left to right. That is to say, only addition and subtraction, or only multiplication and division are mixed operations, and their operation order is calculated from left to right. In an expression, if both first-level operation and second-level operation are included, then the second-level operation should be calculated first, and then the first-level operation should be calculated. That is, "multiply first and then divide, then add and subtract", or "multiply first and then divide, then add and subtract" for short.