Train A of length 180m and speed 72 km/hour crosses train B of length 120 m and speed 108 km/hour moving in the opposite direction in :

Trains are running in opposite directions.

Relative speed = (108 + 72) kmph

$=180 \mathrm{kmph}=\left(\frac{180 \times 5}{18}\right) \mathrm{m} / \mathrm{sec} .=50 \mathrm{~m} / \mathrm{sec}$

Sum of the lengths of trains = (180 +120) metre= 300 metre

$\therefore$ Required time $=\left(\frac{300}{50}\right)$ metre $=6$ seconds