Question 631024
The 1st train has a head start of 45 mph * 3 h = 135 mi
The relative speed of the two trains = 75 mph - 45 mph = 30 mph
So it is as if the 1st train is standing still and the 2nd train approaches at 30 mph.
So the total time will be 135 mi/30 mph = 4.5 hours