Answer
90 mph
Work Step by Step
Average velocity=$\frac{\Delta y}{\Delta t}$
$\Delta t=4\,h $ and $\Delta y $ is the displacement from New Orleans to Memphis (which is the same for both trains). Therefore, the average velocity of the first train is equal to the average velocity of the second train.
It is said that the first train travels at a constant velocity of 90 mph which implies that its average velocity is also 90 mph.
$\implies $ Average velocity of the second train=90 mph
