A train of length 180 metres crosses a tree in 15 seconds and crosses another train of the same length travelling in opposite direction in 20 seconds. What is the speed (in km/hr) of the second train?

Let speed of first train = $$x$$ m/s and speed of second train = $$y$$ m/s

Length of both trains = 180 m

Using, time = distance/speed

=> $$x=\frac{180}{15}=12$$ m/s

The trains are moving in opposite direction, thus relative speed = $$(x+y)=(12+y)$$ m/s

and distance = $$180+180=360$$ m

=> $$12+y=\frac{360}{20}=18$$

=> $$y=18-12=6$$ m/s

$$\therefore$$ Speed (in km/hr) of the second train = $$6\times\frac{18}{5}=21.6$$

=> Ans - (A)