Aptitude - Problems on Trains - Discussion

Good answer. Thanks all for explaining the answer.

Suppose we have to convert km/hour into m/s.
km/hour = 1000m/hour
i hour = 3600s
km/s = 1000m/3600s = 5/18.

Just reverse it, m/s into km/hour = 18/5, it's an easy way.

Someone, please explain this question properly.

@All

Here they mentioned the relative time of the train to the man is 10 seconds which means the train's length(i.e 125m) has just passed the moving man in 10 seconds,so we need to find the relative speed for the train to the man as mentioned below.

i.e, 125/10 =25/2 m/s as it is in m/s,we need to convert into km/hr
25/2 x 18/5 = 45 km/hr (relative speed between train and man)

Next we need to find the original speed of the train that passes the man's speed with the help of the relative speed between the train and man i.e,45 km/hr(,if the Direction of the man and train is opposite to one another, then do addition, if both man and train running in same direction subtract both )
Here man and train are in the same direction.
So subtract;
Let x be the speed of the train and we know the man's speed as 5 km/hr and the relative speed is 45 km/hr.
Since same direction subtraction,
x = 45+5
x = 50 km/hr (answer)
By checking:
Train's speed =50 km/hr ,,man's speed = 5 km/hr, same direction so do subtraction;
50 -5 = 45 to get the relative speed of both.

How it is 50? please explain to me.

Why commonly mention that 5/18? Please explain.

It is simple guys.

If we observe hint, her there should be the relative speed because the train and are travelling in the same direction so we (train) -v (man) T =Distance.

(120/10) (18/5) =v (t) -5.
45+5=v (t).
50=v (t).

Given man's speed = 5km per hour.
The train length is 125m.
The train crosses man in 10 seconds.

It means both the train and the man are moving in the same direction.

The speed related to man is 125m/10sec, which is 12.5m/s.

{convert this into km/hr:- [basic idea:-km/hr to m/s or viceversa;
we know that:1km=1000m, 1hr = 3600sec,
1km/hr to m/s:-1000/3600 = 5/18m/s.
For n*km/hr to m/s = n * (5/18)m/s.
And for viceversa(m/s to km/hr) :n*(18/5)km/hr}.

Now, convert 12.5m/s into km/hr:-12.5*18/5 = 45km/hr.
This is nothing but relative speed, according to man, is 45km/hr.

In basic;

Relative speed = |speed of object1 - speed of object2|(if in the same direction).
Relative speed = |speed of object1 + speed of object2|(if in opposite direction).

The problem is in the same direction,
So;
45 = speed of train - speed of man.
Therefore, the speed of the train = 45 + 5 = 50km/hr.

@Rahul.

Great work, Thanks for explaining.

Very Good Thanks.