1. This rule is the actual need of life. Look at two examples first. Example 1: Xiaohong went to the stationery store and bought four pencils at 30 cents each and six exercise books at 45 cents each. How much did Xiao Hongyi spend? According to the meaning of the question, we should first figure out how much it costs to buy four pencils and six exercise books respectively, and then add up the money spent on buying pencils and exercise books. The formula is 0.3× 4+0.45× 6. This question should be "multiply first and then add". Example 2: The school bought 250 books and distributed them to 4 classes with 45 books in each class. How many books are left? According to the meaning of the question, first calculate how many books are assigned to four classes, that is, how many are four 45, and then subtract the number of books assigned to four classes from 250. The formula is 250-45× 4. This problem should also be "multiplied first and then subtracted". The above example shows that people encounter many problems of "multiplication and division before addition and subtraction" in their daily life, so they make such a rule. Of course, we sometimes encounter the problem of "adding, subtracting, multiplying and dividing first" in our life. In this case, as long as parentheses are added to the addition and subtraction part of the formula, we can calculate what is in parentheses first.

2. This provision can simplify the calculation. From the development of mathematics, multiplication and division are more advanced than addition and subtraction, multiplication is a simple addition and subtraction of the same number, and division is a simple addition and subtraction of the same number. In order to improve the calculation speed, it is necessary to stipulate "multiply first and then divide, then add and subtract". Let's take Example 2 above as an example. If we use addition to find the number of four classes: 45+45+45+45, and then subtract the sum of four 45 from 250 books, it is obviously more troublesome than 250-45 × 4.

Therefore, it is reasonable to stipulate "multiply first, then divide, then add and subtract".