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A tank can be filled by a pipe in 20 min. and by another pipe in 60 min. Both the pipes are kept open for 10 min. and then the first pipe is shut off. After this, the tank will be completely filled in__________?

Correct answer: B. 20 min

  • A. 10 min
  • B. 20 min
  • C. 30 min
  • D. 40 min

Explanation

For the first 10 minutes, the two pipes fill 10 x (1/20 + 1/60) = 10/15 = 2/3 of the tank. The remaining 1/3 is filled by the second pipe at 1/60 tank per minute, requiring (1/3) / (1/60) = 20 minutes. The likely mistake is option c, 30 minutes, from using the combined rate after the first pipe has been shut.

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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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