Question

A tap can fill the tank in 40 min. 80 min will take to empty the tank through the outlet. If both the tap and outlet are opened, the tank will be filled in minutes.

Answer 1

Suppose 1 unit of work is done is 40 minutes,
40 minutes $\to$ 1 unit of work
$1\to \frac{1}{40}$
80 minutes $\to$ 1 unit of work
$1\to \frac{1}{80}$
$t[\frac{1}{40}-\frac{1}{80}]=1$
$t\left[\frac{1}{80}\right]=1\Rightarrow t=80minutes$.