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To fill a cistern, pipes P, Q & R take 20, 15 & 12 minutes respectively. The time in minutes that the three pipes together will take to fill the cistern is:__________?

Correct answer: A. 5 min

  • A. 5 min
  • B. 10 min
  • C. 15 min
  • D. 20 min

Explanation

The combined filling rate is 1/20 + 1/15 + 1/12 = 3/60 + 4/60 + 5/60 = 12/60 = 1/5 cistern per minute. Therefore, the three pipes fill the cistern in 5 minutes. The likely trap is 10 minutes, from combining only two of the pipes or averaging their times.

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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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