Pipes A and B can fill a tank in 10 hours and 12 hours, while pipe C can empty it in 20 hours. If all three are opened on an empty tank, how long will it take to fill?
Correct answer C. 7 hours 30 minutes
Explanation
Net rate = 1/10 + 1/12 − 1/20 = (6 + 5 − 3)/60 = 8/60 per hour. Time = 60/8 = 7.5 hours = 7 hours 30 minutes.
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