GMAT
题目
A small water pump would take 2 hours to fill an empty tank. A larger pump would take hour to fill the same tank. How many hours would it take both pumps, working at their respective constant rates, to fill the empty tank if they began pumping at the same time?
解析
小水泵的效率是\(\frac{1}{2}\)水箱/小时（效率是时间的倒数）；
大水泵的效率是2水箱/小时；
两台水泵的合效率为\(\frac{1}{2} + 2 = \frac{5}{2}\)水箱/小时，因此它们一起将空水箱注满所需的时间为\(1÷(\frac{5}{2}) = \frac{2}{5}\)小时（时间=工作量÷效率）。
答案：C
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