Professor Su is a famous mathematician in China. Once he went to Germany and met a German mathematician. The mathematician solved a difficult problem on the tram and asked Sue to solve it. The topic is:

Party A and Party B set out from two places at the same time, face to face, with a distance of100km. A walks 6 kilometers per hour to 4 kilometers per hour. A take a dog, and the dog walks 10 km every hour. The dog set off with A. When it met B, it turned around and went to A.. After he met A, he went to B until they met. How many kilometers did the dog walk?

The dog ran 100/(6+4)= 10 hour, so it ran 100 km.