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送交者: newbigman 2013月06月12日14:56:25 于 [灵机一动] 发送悄悄话

回答: The reason? 由 gugeren 于 2013-06-12 14:37:19

1) assume that velocity of the river and boat is V0 and V1, respectively.

2) The boat reached (V1-V0)*1 after the first hour

3) Take the boat T= (2+ V1-V0)/(V1+V0) +1, to reach back the starting point.

3) T * V0 = 2

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