Aptitude - Problems on Trains - Discussion

Guys I have one doubt for finding out the relative speed we should need to subtract but here it's divided please explain me how?

18/5 is to convert seconds to hours.

How can be its 50 ?

It's 45 km/hr.

If we put answer 45 km as speed of train.

Time = 125/45*5/18 = 10 m/s.

But if we 50 we can't get its time. 10 m/s.

Explain someone ?

A train speeds past a pole in 15 seconds and a platform 100 m long in 25 seconds. Its length is:

Kindly solve it with full description.

Nice answer with explanation. It is very easy to understand.

Why did we not converted 125 m to km?

Nice explanation.

Dear Friends,
Understand the question:
Train speed relative to man (X-Y) = ?
X is Train speed.
Y is Man speed (given as 5 km/hr).

We have length(distance) train passes = 125m.
Train passed in time = 10 seconds.

Hence, speed = distance/time.
speed = 125/10 meters/seconds.
Speed = 25/2 m/s.

Let's convert m/s to km/hr.

****Remember While conversion of km/hr to m/s, multiply with 5/18 or 0.277.
Similarly, conversion m/s to km/hr, multiply with 18/5 or 3.6.
(these are not blind values, we can get if we calculate)****

= 25/2 X 18/5 km/hr.
= 45 km/hr.

Let's see,
Train speed relative to man.
X-Y = ?
X-Y = 45 km/hr.
as we know Y = 5 km/hr.
X-5 = 45.
X = 45+5 = 50 km/hr.

Guys, we easily get 45 km/hr then we subtract from 45-5 then we get an answer 40 km/hr.

Let the speed of the train be x km/hr. Then, relative speed = (x - 5) km/hr.

x - 5 = 45.
x = 50 km/hr.

Can someone please tell me? When we make the use of relative speed?