Two taps can separately fill a cistern in 10 minutes and 15 minutes respectively. If these two pipes and a waste pipe are kept open simultaneously, the cistern gets filled in 18 minutes. The waste pipe can empty the full cistern in

Let, the time taken by waste pipe to empty the cistern = '$$x$$'

Part filled by two pipes in 1 minute is 1/10 and 1/15 respectively and the cistern gets filled in 18 minutes (given).

Net part filled in 1 hour is given by,

$$\frac{1}{10} + \frac{1}{15} - \frac{1}{x} = \frac{1}{18}$$

$$\frac{1}{10} + \frac{1}{15} - \frac{1}{18} = \frac{1}{x}$$

$$\frac{9 + 6 - 5}{90} = \frac{1}{x}$$

$$\frac{10}{90} = \frac{1}{x}$$ or $$x = 9$$ minutes

Hence, option D is the correct answer.