A train 270 metres long is running at a speed of 36 km per hour n it will cross a bridge of length 180 metres in :

$$\eqalign{ & {\text{Crossing time }} = \frac{{{l_1} + {l_2}}}{{{\text{Speed}}}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{270 + 180}}{{36 \times \frac{5}{{18}}}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{450}}{{10}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 45{\text{ seconds}} \cr} $$