Two pipes A and B can fill a tank in 20 minutes and 60 minutes respectively. There is an outlet pipe C at the bottom of the tank. If all the three pipes are opened together it took 40 minutes to fill the tank. What is the time taken by outlet C to empty the full tank working alone ?

24 min

The correct answer is option (2) : 24 min

Part of tank filled by pipe A in 1 min $+\frac{1}{20}$

Part of tank filled by pipe B in 1 min $=\frac{1}{60}$

Let the time taken by putlet C to empty the full tank be x min, then

Part of the tank emptied by outlet C in 1 min $=\frac{1}{x}$

According to Question

$\frac{1}{20}+\frac{1}{60}-\frac{1}{x}=\frac{1}{40}$

$\frac{4}{60}-\frac{1}{x}=\frac{1}{40}$

$\frac{1}{x}=\frac{1}{15}-\frac{1}{40}$

$x= 24\, min $