Two pipes fill a tank in $12$ and $15$ minutes. If both are opened together and after $3$ minutes the first is closed, the extra time taken by the second pipe to fill the tank is:

Question

Two pipes fill a tank in $12$ and $15$ minutes. If both are opened together and after $3$ minutes the first is closed, the extra time taken by the second pipe to fill the tank is:

Option A

7.5 minutes

Option B

8 minutes

Option C — Correct answer

8.25 minutes

Correct choice.

Option D

9 minutes

Explanation

Take tank capacity $=60$ units. In 3 minutes, both fill $3(5+4)=27$ units. Remaining work $=33$ units. The second pipe fills $4$ units per minute, so extra time $=\frac{33}{4}=8.25$ minutes.

Chapter Mock 7: Time and Work, Pipe and Cistern