Two trains are running in opposite directions with the same speed. If the length of each train is 120 metres and they cross each other in 12 seconds then the speed of each train (in km/hr) is:

Explanation:

Let the speed of each train be $ x $ m/sec.

Then, relative speed of the two trains = 2$ x $ m/sec.

So, 2$ x $ =$ \dfrac{(120 + 120)}{12} $

$\Rightarrow 2 x $ = 20

$\Rightarrow x $ = 10.

$\therefore$ Speed of each train = 10 m/sec =$ \left(10 \times\dfrac{18}{5} \right) $km/hr = 36 km/hr.