You vs. Train (Solution)

He can run 1/4 of the bridge’s distance by the time the train will be at the start of the bridge. So, if he runs away from the train, he will be halfway across the bridge by the time train is starting to cross the bridge. Then, if he continues to run away from the train, he just barely makes it off, which means he runs 1/2 the bridge’s distance in the time it takes the train to travel the full distance. Therefore, he runs half as fast as the train.