Exams › SSC CGL (Prelims) › General

Tap X and Tap Y can fill a cistern in 12 hours and 15 hours respectively. An outlet pipe Z can empty it in 20 hours. If all three are opened together, how long will it take to fill the empty cistern?

1. 8 hours
2. 12 hours
3. 10 hours
4. 15 hours

Correct answer: 10 hours

Solution

Tap X fills \(\frac{1}{12}\) cistern per hour and Tap Y fills \(\frac{1}{15}\) per hour, while Z empties \(\frac{1}{20}\) per hour. Net rate = \(\frac{1}{12}+\frac{1}{15}-\frac{1}{20}=\frac{1}{10}\) cistern per hour, so the cistern fills in 10 hours.

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