Two students clean 5 rooms in 4 hours. How long do 40 students need for 40 rooms? A class decides to do a community involvement project by cleaning classrooms in a school. If 2 students can clean 5 classrooms in 4 hours, how long would it take for 40 students to clean 40 classrooms?

Hint:  the theory is that everybody can work in parallel with full efficiency.  So if you put $40$ students on those $5$ classrooms, how long would it take?  Then you have $8$ times that many classrooms.....
