Multiple choice

A is twice as efficient as B and half as efficient as C. If A takes 21 days to complete the work, find the number of days to complete the work if they work together.

  1. 12.5

  2. 10.5

  3. 8.25

  4. 4.75

  5. None of these
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E Correct answer
Explanation

A takes 21 days, so A's efficiency is 1/21 work per day. Since A is twice as efficient as B, B takes 42 days (efficiency 1/42). Since A is half as efficient as C, C takes 10.5 days (efficiency 2/21). Together, their combined efficiency is (1/21 + 1/42 + 2/21) = 7/42 = 1/6 work per day, so they complete the work in 6 days. None of the given options (12.5, 10.5, 8.25, 4.75) match 6 days.

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