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Bilal can do a work in 15 days and Jalal in 12 days. If they work on it together for 4 days, the fraction of work that is left is________?

Correct answer: C. 2/5

  • A. 3/5
  • B. 4/7
  • C. 2/5
  • D. 1/4

Explanation

Their combined rate is 1/15 + 1/12 = 3/20 work per day, so in 4 days they complete 4 × 3/20 = 3/5 of the work. The fraction left is 1 - 3/5 = 2/5. The likely mistake is 3/5, which is the fraction completed rather than the fraction remaining.

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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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