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Pipes A and B can fill a cistern in 8 and 24 minutes respectively. They are opened an alternate minutes. Find how many minutes, the cistern shall be full?

Correct answer: B. 12 min

  • A. 6 min
  • B. 12 min
  • C. 19 min
  • D. 10 min

Explanation

Assuming A is opened first, in each 2-minute cycle the pipes fill 1/8 + 1/24 = 1/6 tank, so 10 minutes fills 5/6 tank. In the 11th minute A adds 1/8 tank, leaving 1/24 tank, which B fills in 1 minute, making the total 12 minutes. The likely mistake is choosing 10 minutes by stopping after five complete cycles and overlooking the remaining 1/6 tank.

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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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