Question
Two pipes A and B can fill a tank in 15 hours and 20
hours, respectively. Another pipe C can empty the tank in 30 hours. If all three pipes are opened together, how long will it take to fill the tank?Solution
Work done by A in 1 hour = 1/15. Work done by B in 1 hour = 1/20. Work done by C (emptying) in 1 hour = −1/30. Net work done in 1 hour = 1/15 + 1/20 − 1/30. LCM of 15, 20, and 30 is 60. Net work done in 1 hour = (4 + 3 − 2)/60 = 5/60 = 1/12. Time taken to fill the tank = 12 hours. Thus, the correct answer is 12 hours . Ans. a
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