Personal Comment: Find relative speed to do this problem which is more simple than it seems!
How long is the train Enigma
What we know: A train moving at 45 miles per hour meets and is passed by a train going 36 mph the other way. A passenger in the first train see the second train takes 6 seconds to pass him.
Enigma: How long is the second train?
Solution: The speed of the passenger in the first train relative to the second car is 45 + 36 = 81 mph. Then you take the number of feet in a mile(5280) and multiply it by 81. Then you must convert the time to seconds by dividing the product by 3600(the amount of seconds in one hour). Your speed per second is your resultant(118.8 feet per second.) It takes 6 second for the second train to pass so the length of the second train is 6 X 118.8 = 712.8 feet long is the length of the second car. Or you could say .135 miles.
