Two trains 120 m and 180 m long are running into opposite direction on  a parallel track at the speed of 62 km/hr and 82 km/hr respectively . How much long the faster train will take to cross the slower train ( in sec. ) ? 

7.5

Total distance = 120 + 180 = 300 m 

When trains moves in opposite direction, then relative speed = (62 + 82) = 144 km/hr  = 144× \(\frac{5}{18}\) m/sec  = 40  m/sec

Distance = Speed  × Time 

Time taken = \(\frac{300}{40}\) = 7.5 sec.