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?

A student-hour is a unit of work.  It represents 1 student working for an hour, or 60 students working for one minute, or 3600 students working for 1 second, or ...
You're told that cleaning 5 classrooms takes 2 students 4 hours, or $8$ student-hours.  So one classroom takes $\frac{8}{5}$ or $1.6$ student-hours.
So the 40 classrooms will take $40 \times 1.6$ or $64$ student-hours.
The forty students will put out $64$ student-hours in $\frac{64}{40}$ or $1.6$ hours...
