A train running at speed of 84 km/hr passes a man walking in opposite direction at speed of 6 km/hr in 4 seconds. What is length of train (in meter)?

$$\eqalign{ & {\text{Let length of train }} \cr & {\text{ = }}l\,{\text{metre}} \cr & \Rightarrow {\text{Time }} \cr & {\text{ = }}\frac{{{\text{total distance}}}}{{{\text{relative speed in opposite direction}}}} \cr & \Rightarrow 4\sec \, = \,\frac{{l + 0}}{{\left( {84 + 6} \right) \times \frac{5}{{18}}{\text{m/s}}}} \cr & \Rightarrow 4\, = \frac{l}{{90 \times \frac{5}{{18}}}} \cr & \Rightarrow \,l\, = \,100\,{\text{m}} \cr & \therefore {\text{ length of the train = 100 m}} \cr} $$