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Two pipes A and B can fill a tank in 4 and 5 hours respectively. If they are turned up alternately for one hour each, the time taken to fill the tank is?

Correct answer: B. 4 hrs 24 min

  • A. 2 hrs 15 min
  • B. 4 hrs 24 min
  • C. 5 hrs
  • D. 3 hrs

Explanation

In each two-hour cycle, pipe A fills 1/4 of the tank and pipe B fills 1/5, so together they fill 9/20 in 2 hours. After 4 hours, two cycles fill 9/10, leaving 1/10; pipe A fills that in (1/10) ÷ (1/4) = 2/5 hour = 24 minutes, for a total of 4 hours 24 minutes. The likely trap is 5 hours, which assumes the alternation continues without checking the remaining fraction.

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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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