Two pipes A and B can fill a cistern in 15 hours and 20 hours respectively. A third pipe C can empty it in 30 hours. If all three work together, how long to fill the cistern?
A10 hours
B12 hours
C15 hours
D18 hours
Explanation
Net filling rate = 1/15 + 1/20 – 1/30 = (4 + 3 – 2) / 60 = 5 / 60 = 1 / 12 per hour. Time taken = 12 hours.
Exam Relevance
- Topic: Work and Time
- Subtopic: Pipes and Cisterns
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