1, assuming that there were x boys and y girls last semester, then X+Y=750, and 40 students were reduced this semester, that is, Y/5-X/6=40, the equation can be solved, and X=300, Y=450, so there are 360 boys and girls this semester.

2. Assuming that Class One donates X yuan and Class Two donates Y yuan, then X+Y+380=3X and 2Y=X+380 can be listed as equations, so that X=380 and Y=380 can be solved, and the total donation is very simple. Add one and it will come out. 1 140 yuan!

3. Suppose A does X every day and B does Y every day, then X-Y=3, 20X/2= 15Y, and X=9. Y=6 can also be solved from the equations, so what a * * * does is to multiply 9 by 20+6 by 15=270!

Do you understand?