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15 men can build a 500m long wall in 30 days. In how many days can 30 men build a 1.2 km long wall?

Correct answer: D. 36 days

  • A. 48 days
  • B. 24 days
  • C. 40 days
  • D. 36 days

Explanation

The work is proportional to wall length and the number of man-days. For 500 m, the work is 15 men x 30 days = 450 man-days, so for 1,200 m, required work is 450 x 1,200/500 = 1,080 man-days; 1,080 man-days ÷ 30 men = 36 days. Students may choose 48 days by adjusting the wall length but forgetting that the number of men also doubles.

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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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