A and B can together do a piece of work in 15 days. B alone can do it in 20 days. In how many days can A alone do it ?
Correct answer: C. 60 days
- A. 40 days
- B. 50 days
- C. 60 days
- D. 70 days
Explanation
A and B together complete 1/15 of the work per day, while B alone completes 1/20 per day. Therefore A's rate is 1/15 - 1/20 = 1/60 of the work per day, so A alone needs 60 days. Students may choose 40 days by subtracting the completion times, 20 days - 15 days, instead of subtracting the work rates.
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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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