The speeds of train A and B are 46 km/h and 62 km/h respectively.  The lengths of A and B are in ratio of 3 : 5.  When running in opposite direction, they cross each other completely in 12 seconds.  How much time (in seconds) will train B take to cross 395 m long tunnel?

36 sec

Relative speed = 46 + 62 = 108 × \(\frac{5}{18}\) = 30 m/sec

Time to cross = 12 sec

Total distance = 30 × 12 = 360 m

*(Total distance covered to cross each other = sum of length of trains)*

Therefore,

Ratio of length of train A and B ⇒ 3 :  5 =  8

8R = 360 m ⇒ 1R = 45

Length of train B = 5R = 5 × 45 = 225 m

Now Distance covered by train B to cross the tunnel = 225 + 395 = 620m

Speed of B = 62 × \(\frac{5}{18}\) m/sec

Time to cross = \(\frac{D}{s}\) = \(\frac{620\;×\;18}{62\;×\;5}\) = 36 sec