Two taps can separately fill a cistern in 10 minutes and 15 minutes respectively and when the waste pipe is open, they can together fill it in 18 minutes. The waste pipe can empty the full cistern in?

7 minutes
9 minutes
13 minutes
23 minutes

Correct Answer: 9 minutes

Explanation:

Work done by waste pipe in 1 min. = (1/10) + (1/15) - (1/18)
= (1/6) - (1/18) = 1/9
∴ Waste pipe can empty the cistern in 9 min.

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