Copied from Zhuping’s blog:
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?