Two trains one 120 m and 180 m long are running in opposite direction on a parallel track at the speed of 51 km/hr and 57 km/hr respectively. How long will they take to cross each other?

10 sec

Total distance traveled to cross each other = 120 + 180 = 300m

Relative speed in opposite direction = 51 + 57 = 108 km/hr = 108 × \(\frac{5}{18}\) m/sec = 30 m/sec

Time taken to cross =  \(\frac{Total\;Distance}{Relative\;Speed}\) = \(\frac{300}{30}\) = 10 Sec.