Two stations A and B are separated by a distance that takes 24 hours to traverse by train. Every hour, on the hour, a train leaves from Station A for Station B and from Station B for Station A. In other words, in a 24-hour period, twenty-four trains leave each station at 1-hour intervals at exactly the same time from each station, bound for the other. If you are a passenger going from Station A to Station B in one of these trains, how many trains will you meet coming the other way?
Solution is here