Question 290434
Since the two trains are moving in opposite directions, the passenger on a train will see the other train moving at the speed of (sum of the two trains' speed) or 50 + 50 = 100(mph). Since he sees the other train passing in 4 seconds, which means that in 4 seconds or 4/3600 of an hour, the second train moves for a distance of 100* 4/3600 (mile) or 1/9 mile.
So 1/9 is what you need to fill in the blank.