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?

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
$$\color{red}{\frac{40}{25}=\frac85=1\;\text{hour and $\,36\,$ minutes}}\,$$ 
