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?

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?

Even the jogger is running at a particular speed and is not idle so why are we not considering the distance travelled by the jogger when the train passes him?

Please anyone clarify this.

I also agree with you @Ravi by the time when the train takes 36 sec to cross the jogger. The jogger also traveled some dist at that 36 sec of time. Jogger speed is 9km/hr which is 2. 5m/sec which means joggers must have covered 2. 5x36=90 m of dist that's mean now the train has to cover 90 m more to actually cross that jogger.

Am I correct? Please anyone explain me.

Why 45-9? Explain Please.

@Swetty.

Because we have to calculate the relative speed of both objects.

Train speed = 45 kmph.
Jogger speed= 9 kmph.