Aptitude - Problems on Trains - Discussion

"A jogger running at 9 kmph alongside a railway track in 240 metres ahead of the engine" - This means The engine is 240 metres behind of that person. So the train has to cover that distance (120+240) metres by it's own speed (without considering relative speed as it has not reached that jogger yet) and after that train has to cover that jogger (considering relative speed).

When the train is covering (120+240) metres the jogger has moved some extra distance in forward direction by that time. So thinking in that way is going to be more complex.

Here we have to think of two moving objects - one is the train which is of length 120 metres and of speed 45Kmph. Another is of length 240 metres and of speed 9Kmph in Same Direction. (Like two trains moving in same direction with different speed).

So, total length = 240+120 = 360 metres.

Relative speed = (45-9) Kmph = 36*(5/18) m/s = 10m/s.

Hence time = 360/10 = 36 seconds.

What is the difference between 2nd problem and 10th problem?

5/18 we can get like that.

Suppose speed is 10 km/hr so we can say like that.

(10)*1000/3600 m/second, so 1000/3600 will become 5/18.

Why (36 x 5/18) is used? How you get 5/18?

Why we add track length means because man is running on track? So man gets the track length.

Question asked about at what time train cross man. Man has no length then why we add track length?

Answer is 48 sec as jogger also jogs 2.5x distance.

= 360+2.5x/x = 10.

= 360+2.5x = 10x.

= 360 = 7.5x.

= 48 = x.

Hence 48 sec is the answer.

Please clear my doubt.

Let the train pass after t time.

So train has to cover (120+240+x)/45 in t time.

And jogger covers x/9 in t time.

(120+240+x) /45 = x/9.

Answer is 90 sec but logic is correct that they both meet after t time.

I still can't understand how the 5/18 came about.

Yes @Vedant, I also get the same answer through the same process.