Suppose that Amtrak has trains leaving from Washington to New York City and also from New York City to Washington every hour on the hour. Also suppose that the trip from one city to the other takes 4.5 hours and all trains travel at the same speed. If you are on the train from DC to NYC, how many trains going the other way will you pass?
12:00. That’s 5 so far that you will pass before you reach NYC.
4:00. That’s 4 more. So you’ll pass 9 NYC-DC trains.