Pipes A and B fill a tank in 30 minutes and 20 minutes; pipe C empties it in 60 minutes. A and B are opened on an empty tank, and C is opened 6 minutes later. What is the total time to fill the tank?
Correct answer B. 13.5 minutes
Explanation
In 6 minutes A and B fill 6 × 1/12 = 1/2. Then the net rate is 1/12 − 1/60 = 1/15, so the other half takes 7.5 minutes. Total = 13.5 minutes.
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