Question

Four pipes can fill a tank in $70$ minutes. How long will it take to fill the tank if $7$ pipes are used?

Answer 1

Let $7$ pipes take $x$ minutes to fill the tank.
This is a case of inverse variation.

More pipes means less time to fill the tank.
$\therefore 7\times x=4\times 70$

$\Rightarrow x=\frac{4\times 70}{7}=40$
Hence, the tank will be filled in $40$ minutes.