The distance between a and B is 22.5km, and they are facing each other at the speed of 2.5km and 5km per hour respectively. Meanwhile, the dog with a runs to B at the speed of 7.5km per hour. The dog runs back to a immediately after meeting B, and then runs to B again after meeting a Until a, B meet, then the dog's journey is______ ．

Suppose that the dog's distance is x kilometers, according to the meaning of the question: x7.5 = 22.52.5 + 5, the solution is: x = 22.5 (kilometers). Answer: the dog's distance is 22.5 kilometers

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