A cistern has two taps which fill it in 12 min. and 15 min. respectively. There is also a waste pipe in the cistern. When all the pipes are opened the empty cistern is full in 20 min. how long will the waste pipe take to empty a full cistern?

8 min.
10 min.
12 min.
16 min.

Correct Answer: 10 min.

Explanation:

Work done by waste pipe in 1 min. = (1/12 + 1/15) - 1/20
= (3/20 - 1/20) = 1/10
∴ Waste pipe can empty the cistern in 10 min.

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