CBSE Class 11-science Answered

velocity of mail train u₁= 108Km/hr = 30m/s
velocity of passenger train u₂= 36m/s = 10m/s
Acceleration =a= -1m/s²
Distance between the two trains = 250m

According to the equation of motion: 2aS = v₁ ²-u₁ ² 
              Where v ₁   = 0 m/s    (train stops after brakes)

Hence we get : S = 450m/s that is the distance moved by the mail train before stopping                                        
 S = u ₁ t   + 1/2 at²

where t is the time taken to stop.: t = 30sec

In the same time the passenger train travels a distance of 30 x 10 = 300m.

The total distance between the passenger train and mail train is 300+250m = 550m but the mail train travels only 450 m . hence the collision is saved.