Pipes A and B can fill a tank in 6 hours and 9 hours respectively. They are opened alternately for one hour each, starting with A. In how many hours will the empty tank be full?
Correct answer C. 7 hours
Explanation
Every 2 hours the pipes fill 1/6 + 1/9 = 5/18. After 6 hours 15/18 is full; A fills the last 3/18 = 1/6 in the 7th hour, total 7 hours.
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