Two pipes A and B can fill a tank in 32 minutes and 48 minutes respectively. If both the pipes are opened simultaneously, after how much time B should be turned off so that the tank is full in 20 minutes?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 18 minutes
Pipe A fills \(\frac{1}{32}\) of the tank per minute and stays open for all 20 minutes:
\( \displaystyle \text{part filled by A} = \frac{20}{32} = \frac{5}{8} \)
The remaining part must come from B:
\( \displaystyle 1 - \frac{5}{8} = \frac{3}{8} \)
Pipe B fills \(\frac{1}{48}\) of the tank per minute, so it must run for
\( \displaystyle \frac{3}{8}\times 48 = 18 \text{ minutes} \)
So B should be turned off after 18 minutes.