Pipes & Cisterns · Question 30 · Hard
Pipes & Cisterns question 30
A cistern has a small leak. Without the leak, a pipe fills it in 6 hours. One day the pipe was opened at 9 AM on the empty cistern with the leak present. The leak was sealed at 12 noon, and the cistern became full at 4 PM. How long would the leak alone take to empty the full cistern?
- 18 hours
- 9 hours
- 24 hours
- 12 hours
Answer and explanation
Answer: A. 18 hours
Explanation: From noon to 4 PM the pipe alone fills 4/6 = 2/3. So from 9 AM to noon (3 hours) the net filling was 1/3, i.e. 1/9 per hour. Leak rate = 1/6 − 1/9 = 1/18, so it empties the cistern in 18 hours.
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