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Pipes A and B can fill a tank in 12 and 15 hours respectively. Pipe C empties it in 20 hours. With all three open, the tank fills in

A12 hours
B15 hours
C8 hours
D10 hours
Answer & Solution
Correct answer: D. 10 hours
Net rate = 1/12 + 1/15 - 1/20. LCM = 60: 5/60 + 4/60 - 3/60 = 6/60 = 1/10. Time = 10 hours.
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