GMAT
题目
Working together at their respective constant rates, Bob and Sam can mow a lawn in 12 hours. If Bob’s rate is twice Sam’s rate, how many hours would it take Bob, working alone, to mow the lawn?
解析
\(\frac{1}{b} + \frac{1}{s} = \frac{1}{12}\)
\(\frac{1}{b} = \frac{2}{s}\)
可得 \(2b = s\)
代入得：
\(\frac{3}{2b} = \frac{1}{12}\)
求解得：\(b = 18\)
因此 \(s = 36\)
答案是B
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