I haven't learned how to formulate the unknowns in the second volume of mathematics in the fourth grade of primary school.

Different types of questions are answered in different ways! Take the topics you listed as an example:

"The sum of A and B is 149, and A is 8 times more than B; What are A and B? "

If A is exactly 8 times that of B, the sum of A and B is 149-5 = 144.

At this time, if B is regarded as 1, A is 8, and the sum of A and B is1+8 = 9;

Divide the total sum by the total number of copies to calculate a copy, that is, b is:144/9 =16;

A is eight times more than B: 8* 16+5 = 133.