GMAT
题目
A company has two types of machines, type R and type S. Operating at a constant rate, a machine of type R does a certain job in 36 hrs and a machine of type S does the same job in 18 hours. If the company used the same number of each type of machine to do the job in 2 hours, how many machines of type R were used?
解析
A机器的速率为\(\frac{1}{36}\)任务/小时，\(x\)台A机器的速率为\(\frac{1}{36}x\)任务/小时；
B机器的速率为\(\frac{1}{18}\)任务/小时，\(x\)台B机器的速率为\(\frac{1}{18}x\)任务/小时（两种机器数量相同）；
速率可以相加，因此A、B机器的合速率为：
\(\frac{1}{36}x + \frac{1}{18}x = \frac{3}{36}x = \frac{1}{12}x\)任务/小时
已知数量相同（即各\(x\)台）的A、B机器合作完成1项任务需要2小时，根据“时间×速率=工作量”：
\(2 × \frac{1}{12}x = 1\)，解得\(x = 6\)
答案：C
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