数学题:一艘船在静水中每小时行驶40千米，水流的速度是5千米/小时。这艘船从甲地顺流而下，7小时到达乙地。如果该船从乙地逆流而上，需要几小时到达甲地?

数学题解答：顺流而下，实际的行驶速度是船的速度+水流的速度，先根据这一点算出两点的距离：

(40 + 5) × 7 = 315（千米）

逆流而上，实际速度则是船速减去水流速度，需要的时间是：

315 ÷ (40 – 5) = 9（小时）

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