Description for Correct answer:
Pipe A and Pipe B together fill \( \Large \frac{1}{10}+\frac{1}{15}=\frac{5}{30}=\frac{1}{6} \)th of the tank in a minute

Therefore, the man would have returned to the tank 6 minutes after opening the taps hoping to see the tank full. However, he finds Pipe C open and shuts it immediately.

After shutting Pipe C, the tank was full in 2 minutes.

Therefore, in the last two minutes \( \Large \frac{1}{6} \times 2=\frac{1}{3} \)rd of the tank would have been filled by Pipe A and Pipe B.

This is the quantum of water that Pipe C would have drained during the first 6 minutes.

lf Pipe C drained 1/3rd of the tank in 6 minutes, then it will take 18 minutes to drain the entire tank.