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Copied from Zhuping’s blog:
一次，诺贝尔经济学获奖者数学家约翰·纳什参加一个数学年会，会间休息的时候，有人就给他出了一道数学题。有甲、乙两人分别从两个地点出发往中间走，这两个地点相距 20公里，甲、乙两人的速度都是每小时10公里。甲还带了一条狗，这条狗的速度是每小时30公里，它先是和甲一起出发向乙走，遇到乙后就折返向甲走，遇到甲后再折返向乙走，如此反复直到甲、乙在中间会合。现在问当甲、乙在中间会和时，狗走了多远。
Two people (guy A and guy B), who are 20 km away from each other, are walking toward each other, at 10 km per hour. There is a dog, which goes at 30 km per hour. She (the dog) started with the guy A first. Because she goes faster than A, so she will meet B on the way. When she meet the B, she will turn around to run toward A, and so on,…until A and B meet. The question is: how much distance did the dog take before A meet B?