A boy walks out on a train trestle high above a river. He has walked 4/7 of the way across the trestle when he spots a train coming at him head-on. He realizes that he has just enough time either to run toward the train and jump off the trestle or to run away from the train and get off the trestle. If the boy can run 6 miles per hour, how fast is the train moving toward him?
Can you answer without writing anything down?