Question

Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. if the length of each train be 120 metres, in what time (in seconds) will they cross each other travelling in opposite direction? select one:
a. 8 sec
b. 12 sec
c. 15 sec
d. 10 sec

Answer 1

Speed of 1st train = Distance / Time = 120 m / 10 s = 12 m/s
Speed of 2nd train = Distance / Time = 120 m / 15 s = 8 m/s

Time = Relative distance / Relative Velocity
= (120 m + 120 m) / (12 m/s + 8 m/s)
= 12 seconds

They’ll cross each other in 12 seconds

[b] is the correct option.