Question

A and B can do a job together in 12 days. If A is 2 times as efficient as B, then how many days shall B alone take to complete the work?
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Answer 1

If they can finish together in 12 days, then:

1/A + 1/B=1/12

Since A is twice efficient as B, then 1/A=2/B. So:

2/B + 1/B=1/12

3/B=1/12

B=36

A=B/2=18

A takes 18 days alone to finish; B takes 36 days alone to finish