Two identical trains A and B running in opposite directions at the same speed take 2 min to cross each other completely. The number of bogi…

Let the length of each bogie be L, and their velocities be v each.

Initial length of each train = 12L.

Whenever a train crosses another train, each train travels a distance equal to the sum of their lengths at relative speed.

∴ in this case, distance traveled = 12L + 12L = 24L

Since the trains are running in the opposite directions, we have

Relative speed = v + v = 2v

Time taken to cross each other = 24L/2v = 12L/v

⇒ 120 = 12L/v

⇒ L/v = 10

Now, given that the length of train A increased by 4 bogies ⇒ New Length of A = 16L

∴ New distance to be traveled = 16L + 12L = 28L

Hence, time taken now to cross each other = (16L + 12L)/2v = 14L/v = 140 sec

Hence, the time would increase by 20 sec.