Time elapsed from the time the trains first begin to overlap to the time they have completely passed one another= 1.875 s
Explanation:
Relative velocity of two trains moving in opposite directions is given by
Vr= V1 + V2
V1= velocity of first train=50 ft/s
V2= Velocity of second train=30 ft/s
so Vr= 50+30= 80 ft/s
distance = d= 150 ft
so the time taken= distance/ relative velocity
t=150/80
t=1.875 s