Pipes A and B fill a tank in $20$ and $30$ minutes respectively. They are opened together, and the tank is filled in $18$ minutes because A was stopped after some time. After how many minutes from the start was A stopped?

Question

Pipes A and B fill a tank in $20$ and $30$ minutes respectively. They are opened together, and the tank is filled in $18$ minutes because A was stopped after some time. After how many minutes from the start was A stopped?

Option A

6 minutes

Option B — Correct answer

8 minutes

Correct choice.

Option C

10 minutes

Option D

12 minutes

Explanation

B works for all 18 minutes and fills $\frac{18}{30}=\frac35$ of the tank. The remaining $\frac25$ is filled by A, which takes $\frac25\div\frac1{20}=8$ minutes. So A was active for 8 minutes from the start.

Chapter Mock 7: Time and Work, Pipe and Cistern