Aptitude - Problems on Trains - Discussion

Because if two trains run in the same direction the Speed of train will decrease to pass another train.

@All.

Ahead means behind that's why we should minus the speed because they are in the same direction.

Why we consider the length of the track?

The total distance covered by the train will be 288m with its original speed. Thereafter relative velocity will take place. So, the actual answer is 35.04 sec

In this question, we have been asked to find the time taken by the train to pass the jogger when it is given that the jogger’s speed is 9 kmph and the train’s speed is 45 kmph and the jogger is 240 m ahead of a 120 m long train. To solve this question, we should know about relative speed, which is the speed of one object with respect to the other and when both objects are moving in the same direction, then their difference is taken as their relative speed. So, we can say that the relative speed of the train with respect to the jogger is, the speed of the train - speed of the jogger.

So, we get,

Relative speed of train = (45 - 9)kmph = 36 kmph.

So, if we consider the relative speed of the train as the original speed, then we have to assume that the jogger is standing. So, we can say that the train must travel a distance of (240 + 120) m = 360 m as the front of the engine and the jogger are 240 m apart and the engine is 120 m long.

Now, we know that 1 km = 1000 m and 1 hr = 3600 sec. So, we can say that, 36 kmph = 36×10003600=10.

Mpsec.

We know that speed, distance, and time are related as, speed=distancetime.
It can also be written as, time=distancespeed.
Now, we know that the train has to travel 360 m with a speed of 10 mpsec. So, we can say that,
Time taken = 36010⇒36sec.
Hence, we can say that a 120 m long train with a speed of 45 kmph at a distance of 240 m from a jogger who is running at a speed of 9 kmph will cross the jogger in 36 sec.
Therefore, the correct option is option C.

Thankyou for explaining the answer.