Two pipes A and B fill a tank in 12 minutes and 15 minutes respectively. If both the pipes are opened simultaneously and pipe A is closed after 3 minutes, then how much more time will it take to fill the tank with pipe B?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 8 minutes 15 seconds
In one minute A fills \( \frac{1}{12} \) and B fills \( \frac{1}{15} \) of the tank.
Work done by both together in 3 minutes:
\( \displaystyle 3\left(\frac{1}{12}+\frac{1}{15}\right)=3\times\frac{5+4}{60} \)
\( \displaystyle =\frac{27}{60}=\frac{9}{20} \)
\( \displaystyle \text{Part left}=1-\frac{9}{20}=\frac{11}{20} \)
Time taken by B alone to fill this part:
\( \displaystyle \frac{11}{20}\times 15=\frac{165}{20}=8.25\ \text{minutes} \)
0.25 minute = 0.25 × 60 = 15 seconds, so B takes 8 minutes 15 seconds more.