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

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