Multiple choice

A, B and C can individually complete a piece of work in 20 days, 15 days and 12 days, respectively. B and C start the work and they work for 3 days and leave. Then, the number of days required by A to finish the remaining work is

  1. 11

  2. 14

  3. 12

  4. 9
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A Correct answer
Explanation

A's rate is 1/20, B's is 1/15, C's is 1/12. B and C work for 3 days: 3 * (1/15 + 1/12) = 3 * (4+5)/60 = 27/60 = 9/20. Remaining work = 11/20. A takes (11/20) / (1/20) = 11 days.

AI explanation

Using the least common multiple method, let the total work be 60 units. The efficiencies of A, B and C are 3, 4 and 5 units per day, respectively. The combined efficiency of B and C is 9 units per day, so in 3 days they complete 27 units, leaving 33 units of remaining work. Dividing the 33 remaining units by A's efficiency of 3 units per day gives 11 days.

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