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 ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 20
In one minute, A fills 1/40 of the tank and B fills 1/60. Let A be closed after t minutes. B runs for the whole 30 minutes.
The parts filled add up to one full tank:
\( \displaystyle \frac{t}{40}+\frac{30}{60}=1 \)
\( \displaystyle \frac{t}{40}=\frac{1}{2},\quad t=20 \)
So pipe A is turned off after 20 minutes.