Description for Correct answer:
The two inlet pipes can fill the cistern in 10 min and 15 min respectively and the outlet pipe can empty at the rate of 5 min.

When all 3 pipes are kept open simultaneously, the quantum of water filled in 1 minute = \( \Large \frac{1}{10}+\frac{1}{15}-\frac{1}{5}=\frac{-1}{30}\ or\ \frac{1}{30} \)th of the tank gets emptied in a minute.

Therefore, the cistern will empty in 30 minutes.