Two pipes A and B can fill a tank in 40 minutes and 60 minutes respectively. Both pipes A and B are opened together. After some time pipe A is closed. If the tank is filled in 30 minutes, then after how many minutes the pipe A is turned off ?

20

The correct answer is Option (3) → 20

Work done by Pipe A in $t$ minutes,

Work By $A = \frac{t}{40}$

Work By $B = \frac{30}{60}=\frac{1}{2}$

$⇒\frac{t}{40}+\frac{1}{2}=1$

$⇒t=20$ minutes