Aptitude - Problems on Trains - Discussion

Speed of jogger=9km/h=9*(5/18)=2.5m/s.
Length=240.

Length of train = 120m.
Speed of train = 45km/h=45(5/18)=12.15m/s.

Relative Length=240+120=360m.
Relative speed=12.15-2.5=9.65m/s.

Speed = Distance/Time.
9.65 = 360/Time.
Time = 360/9.65.
= 36.

When the train is moving towards the jogger the jogger is still running at 9Kmph or 2.5 m/sec so by the time the engine reaches 240mtrs the jogger would've moved forward.

Time taken for front of train to cover 240 mtrs = 24 secs (time = distance / speed{relative = 10 m/s})

Also the distance travelled by jogger in this 24 secs is = 2.5 mtr/sec * 24 sec = 60 mtrs
so in order for the train to completely pass the jogger, it has to travel 60 mtrs more.

Total distance = 60 + 120 = 180 m.
Speed = 10 m/s.
Time = 18 sec.

Total time = 18 + 24 = 42 sec.

Is this method is correct?

Why we have taken the distance covered by train as (120m +240m)? why to add the length of the track? Please explain.

The distance to be covered is the length from the train to the jogger (240) And we are adding 120 because we need to know the time when the whole train passes the jogger.

The jogger is running at 9kmph.

240m ahead from 120m long train.

When the train covers 240m at speed of 45kmph in t = 96/5 sec in the same time the jogger covers 48m.

Since he is jogging at a particular speed so the total distance covered by the train to pass the jogger would be 120+240+48.

Then, how it could be 240 +120?

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