Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train is 120 m, in what time will they cross each other traveling in the opposite direction?

Correct option is B

Given:
Length of each train = 120 m
Speed of first train = 120/10 = 12 m/s
Speed of second train = 120/15 = 8 m/s
Formula Used:

If trains are moving in opposite direction, then their relative speed:
Relative Speed = Sum of speeds

Solution:

Let speed of first and second train be x m/sec and y m/sec respectively.

Length of each train is 120 m.

According to the question

x = 120/10

=> x = 12 m/sec

And, y = 120/15

=> y = 8 m/sec

Relative speed, if they cross each other travelling in the opposite direction = 12 + 8 = 20 m/sec

∴ Both train cross each other = (120 + 120)/(12 + 8) = 240/20 = 12 seconds