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A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 28 minutes. When both pipes are opened, find when the cistern will be full?

Correct answer: C. 70 min

  • A. 60 min
  • B. 80 min
  • C. 70 min
  • D. 48 min

Explanation

The filling pipe supplies 1/20 tank/minute and the leak removes 1/28 tank/minute. Their net rate is 1/20 - 1/28 = 8/560 = 1/70 tank/minute, so the cistern fills in 70 minutes. The likely wrong choice is 48 minutes, which comes from combining the times directly instead of calculating the difference between the rates.

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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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