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?

Let 21 pipes take x minutes to fill the tank.

Since it is a case of inverse variation,

$\therefore 12 \times 42 = 21 \times x\\[1em] \Rightarrow 504 = 21x\\[1em] \Rightarrow x = \dfrac{504}{21}\\[1em] \Rightarrow x = 24$

Hence, 21 pipes will take 24 minutes to fill the tank.