Question

Tap $A$ can fill a tank in $8$ hours while tap $B$ can fill it in $4$ hours. In how much time will the tank be filled if both $A$ and $B$ are opened together?

Answer 1

If $t$ is the time taken to fill the tank when $A$ and $B$ are both open, then
$\frac{1}{t}=\frac{1}{8}+\frac{1}{4}=\frac{3}{8}$.
Hence, $t=\frac{8}{3}$. Thus the time taken to fill the tank when $A$ and $B$ are both open is $2$ hours and $40$ minutes.