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Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in?

Correct answer: C. 3 9/17 hours

  • A. 1 13/17 hours
  • B. 2 8/11 hours
  • C. 3 9/17 hours
  • D. 4 1/2 hours

Explanation

The combined rate is 1/5 + 1/6 - 1/12 = 17/60 tank/hour, where the emptying pipe is subtracted. Hence the filling time is 60/17 = 3 9/17 hours. The likely mistake is selecting 2 8/11 hours by adding the emptying pipe's rate instead of subtracting it.

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About Time and Work

Problems measure work through rates, treating each person's or machine's contribution as work completed per unit time. Questions cover combined work, efficiency, wages, alternate working schedules, pipes and cisterns, and men-days relationships, with care needed to distinguish time taken by one worker from the time required when workers act together.

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