Question

$12$ pipes, all of the same size, fill a tank in $42$ minutes. How long will it take to fill the same tank, if $21$ pipes of the same size are used?

Answer 1

Since $12$ pipes fill the tank in $42$ minutes.

So, $21$ pipes will do that in x minutes.

$\begin{array}{c}12\times 42=21\times x\\ \Rightarrow \frac{12\times 42}{21}=x\end{array}$

$=24$ minutes.