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: 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...
A: 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.....
A: Two students clean $\,5\,$ rooms in $\,4\,$ hours, which means they both clean $\,\frac54\,$ of a room per hour, and this means each student cleans $\,\frac58\,$ of a room per hour.
Thus, $\,40\,$ students clean $\,40\cdot\frac58=25\,$ rooms per hour, which means they all together need for $\,40\,$ rooms
$$\color{red}{\frac{40}{25}=\frac85=1\;\text{hour and $\,36\,$ minutes}}\,$$ 
