I met a three-level question, saying that Party A and Party B were standing at one end of the road, with a total length of 260 kilometers. Party A walks to the other end at the speed of 20km per hour, and Party B walks to the other end at the speed of15km per hour. A There is a dog. At the beginning, run to the second end at a speed of 5 km/h, return when you see B, return when you return to A, return when you see B, and so on until A and B meet. Ask the dog how many kilometers he ran.