Two pipes A and B can fill a tank respectively in 30 min and 45 min. Both A and B are opened together for some time and then pipe B is turned off. If the tank is filled in 20 min, then find after how many minutes the pipe B is turned off?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 15 min
In one minute A fills \(\frac{1}{30}\) of the tank and B fills \(\frac{1}{45}\). Pipe A runs for the whole 20 minutes; let B run for t minutes. Together they fill one tank:
\( \displaystyle \frac{20}{30} + \frac{t}{45} = 1 \)
\( \displaystyle \frac{t}{45} = 1 - \frac{2}{3} = \frac{1}{3} \)
\(t = 15\)
So pipe B is turned off after 15 minutes.