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问答题
A man jogs at 6 mph over a certain distance and walks back over the same route at 4 mph. What is his average speed for the journey?
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参考解析
解析：
mph英里/小时。设单程距离为x，平均速度=总距离/总时间=2x/(x/6+x/4)=4.8
mph英里/小时。设单程距离为x，平均速度=总距离/总时间=2x/(x/6+x/4)=4.8
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