Mahmud6 wrote:

Bunuel wrote:

Official Solution:

If a passenger sitting near the window in a train moving at 40 kmh noticed that it took 3 seconds for the oncoming train to pass by, what was the speed of the oncoming train if the length of the oncoming train was 75 meters?

A. 50 kmh

B. 52 kmh

C. 56 kmh

D. 60 kmh

E. 70 kmh

Denote the speed of the oncoming train as \(V\). Then its speed relative to the passenger is \(V + 40\). 75 meters in 3 seconds is the same as 25 meters in 1 second or 90 kmh. Thus, \(V + 40 = 90\) and \(V = 50\) kmh.

Answer: A

"Then its speed relative to the passenger is \(V + 40\)." ----> Could not understand it. Would you please clarify this a little bit.

This is the concept of Relative Speed.

When two objects (speeds V1 and V2) move in opposite directions (towards each other or away from each other), they cover the distance between them (or create distance between them) at the rate of (V1 + V2).

When two objects move in same direction, their speeds get subtracted.