pipes P and Q would fill a cistern 18 and 24 minutes respectively. Both pipes being opened, find when the first pipe must be turned off so that the cistern may be just filled in 12 minutes?
Correct answer: B. after 9 mins
- A. after 12 mins
- B. after 9 mins
- C. after 8 1/2 mins
- D. after 10 mins
Explanation
Pipe Q remains open for all 12 minutes and supplies 12/24 = 1/2 tank. Pipe P must supply the remaining 1/2 tank, so its operating time is (1/2 tank) ÷ (1/18 tank/minute) = 9 minutes. The likely wrong choice is 8 1/2 minutes, caused by using an inaccurate combined-rate calculation.
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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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