a train of length 100m runs at a speed of 120km/hr from north to south. another train of length 150m travels with a speed of 80 km/hr from south to north. what is the time required to cross each other ?

Answer 1

Let the faster train be train A.
Let the slower train be train B.

The speed of train A relative to train B, and vice- versa, is 120 km/h + 80 km/h =200 km/h.

The distance traversed from when the fronts of the trains meet each other (presumably on separate tracks!) and the backs of the trains depart from each other is the sum of the trains' lengths = 100 m + 150 m = 250 m = 0.25 km
t = d/v
t = 0.25 km / 200 km/h
t = ____

Answer 2

Total length = 100+150= 250m
Total speed =120+80 = 200km/hr= 200*5/18 m/s

Time = 250/(200*5/18) = (250*18)/(200*5) = 4500/1000 = 4.5s