8. If the chief engineer is 1, the first-year students will complete 1/7.5 per hour, and the second-year students will complete 1/5 per hour.

Working together 1 hour, we have already completed1/7.5+1/5 =1/3, and the remaining 2/3 are completed by junior two students. We still need (2/3)/(1/5).

So * * * takes 13/3 hours.

9. Let X people do it for 2 hours, and the equation can be listed according to the meaning of the question.

(x/80)*2+(x+5)/80*8=3/4

X=2 to solve this equation.

So the two men began to sort out the information.