Two pipes A and B can fill a tank in 20 minutes and 10 minutes respectively. Both pipes A and B are opened together for some time and then pipe B is turned off. If the tank is filled in 15 minutes, then find after how many minutes pipe B is turned off?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2.5 minutes
In one minute pipe A fills \(\frac{1}{20}\) of the tank and pipe B fills \(\frac{1}{10}\). Let B be turned off after t minutes. Pipe A stays open for the whole 15 minutes, while B works only for t minutes. The work done by each pipe adds up to one full tank:
\( \displaystyle \frac{15}{20} + \frac{t}{10} = 1 \)
\( \displaystyle \frac{t}{10} = 1 - \frac{3}{4} = \frac{1}{4} \)
\(t = 2.5\)
So pipe B is turned off after 2.5 minutes.