A is thrice as good a workman as B and therefore is able to finish a job in 60 days less than B. Working together, they can do it in:

20 days
22 1/2 days
25 days
30 days

Correct answer: 22 1/2 days

Solution

If B takes $x$ days, then A, being thrice as efficient, takes $\frac{x}{3}$ days. Given $x-\frac{x}{3}=60$, we get $x=90$ and A takes 30 days. Together, their rate is $\frac{1}{30}+\frac{1}{90}=\frac{4}{90}=\frac{2}{45}$, so they finish in $\frac{45}{2}=22\frac{1}{2}$ days.

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