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Pipe A can fill a tank in 16 minutes and pipe B cam empty it in 24 minutes. If both the pipes are opened together after how many minutes should pipe B be closed, so that the tank is filled in 30 minutes?

Correct answer: D. None of these

  • A. 20
  • B. 24
  • C. 23
  • D. None of these

Explanation

During the first t minutes, the net rate is 1/16 - 1/24 = 1/48 tank/minute; after B closes, A supplies at 1/16 tank/minute for 30 - t minutes. Thus t/48 + (30 - t)/16 = 1, giving t + 90 - 3t = 48 and t = 21 minutes, which is not listed. The likely mistake is choosing 20 minutes by rounding or by ignoring the different rates before and after B closes.

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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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