Two trains of equal length take 10 sec and 15 sec respectively to cross a telegraph post. If the length of each train be 120 metres, in what time (in seconds) will they cross each other travelling in opposite direction ?

$$\eqalign{ & {{\text{S}}_1} = \frac{{120}}{{10}}{\text{ m/sec}} \cr & \,\,\,\,\,\,\,\, = {\text{12 m/sec}} \cr & {{\text{S}}_2} = \frac{{120}}{{15}}{\text{ m/sec}} \cr & \,\,\,\,\,\,\,\, = 8{\text{ m/sec}} \cr} $$
Time taken to cross each other :
$$\eqalign{ & {\text{ = }}\frac{{{l_1} + {l_2}}}{{{{\text{V}}_{{\text{rel}}{\text{.}}}}}} \cr & = \frac{{240}}{{20}} \cr & = 12\sec \cr} $$