Two pipes A and B can fill a tank in 20 minutes and 30 minutes respectively. Both pipes A and B are opened together for some time and pipe B is turned off. If the tank is filled in 15 minutes, then after how many minutes pipe B is turned off?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 7.5 minutes
In one minute, pipe A fills \(\frac{1}{20}\) of the tank and pipe B fills \(\frac{1}{30}\). Pipe A runs for all 15 minutes. Let pipe B run for t minutes. The parts filled add up to one full tank:
\( \displaystyle \frac{15}{20} + \frac{t}{30} = 1 \)
\( \displaystyle \frac{t}{30} = 1 - \frac{3}{4} = \frac{1}{4} \)
\(t = 7.5\) minutes