An express train travelling at 80 km/hr overtakes a goods train, twice as long and going at 40 km/hr on a parallel track, in 54 s. How long will the express train take to cross a platform of 400 m long?

- 18 s
- 27 s
- 36 s
- 45 s

**Answer **

Relative speed of two trains = (80 - 40) = 40 km/hr.

At this speed in 54 s, they would travel an effective distance of (40 x 54)/3600 = 0.6 km or 600 m.

This effective distance should be equal to the sum of the lengths of the two trains. So, if length of the express train is L, length of the goods train will be 2L. Hence,

L + 2L = 600 or L = 200 m.

So the time taken by express train of 200 m to cross a platform 400 m long will be = (200+400)/(80 X 5/18) = 27

**The correct option is B.**