2. 外语
  3. Working alone, a small pump takes twice as long as a large pump takes to fill an empty tank. Working together at their respectiv

Working alone, a small pump takes twice as long as a large pump takes to fill an empty tank. Working together at their respectiv

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问题 Working alone, a small pump takes twice as long as a large pump takes to fill an empty tank. Working together at their respective constant rates, the pumps can fill the tank in 6 hours. How many hours would it take the small pump to fill the tank working alone?
选项 A、8
B、9
C、12
D、15
E、18
答案E
解析 单独工作时，一个小水泵充满一个空水箱所花的时间是大水泵的两倍。两个水泵保持其各自通常的速度共同工作时，共需花6个小时灌满水箱。若小水泵单独工作时，需花多长时间才能灌满水箱?
解：本题的正确答案是(E)。设小水泵需花x小时灌满水箱，则大小泵需花x/2小时灌满小箱，由题意可得：(1/x + 2/x)×6=1 → x=18
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