Two trains, one 160 m and the other 140 m long are running in opposite directions on parallel tracks, the first at 77 km an hour and the other at 67 km an hour. How long will they take to cross each other ?

$$\eqalign{ & {{\text{V}}_{{\text{rel}}{\text{.}}}} = 77 + 67 = 144{\text{ km/hr}} \cr & {\text{ = 144}} \times \frac{5}{{18}}{\text{ m/sec}} \cr & = 40{\text{ m/sec}} \cr & \therefore {\text{ T}} = \frac{{\text{D}}}{{{{\text{V}}_{{\text{rel}}{\text{.}}}}}} \cr & \Rightarrow {\text{T}} = \frac{{140 + 160}}{{40}} \cr & \Rightarrow {\text{T}} = \frac{{300}}{{40}} \cr & \Rightarrow {\text{T}} = 7.5\sec {\text{or 7}}\frac{1}{2}\,\sec \cr} $$