Aptitude - Problems on Trains - Discussion

@Harish.

Why do we need the length of train?"
=> when the question says " A train passes a station platform in 36 seconds" it means the time between when an engine of the train has crossed the starting point of platform till trailer had passed the end point of the platform.

If you actually analyse this, we find that train had already passed platforms total length(x) when its engine crossed end point of platform and it is still going on till its trailer crosses end point of platform(i.e. train went 300m extra which is its own length)
Trains pass length = x+300;
Trains pass time= 36 sec.

Therefore we substitute (distance) = (speed) * (time).
(x+300)= (54 * 5/18) * 36.
=> x =240m.

Time ratio of train A & B is 27 : 17.
Hence speed ratio of A & B respectively is 17:27.
They meet after 23 sec ,hence 23-17 =6 & 27-23 =4.
Then ratio is 6 : 4 means 3 : 2.

If you can think then,

(36-20) * 54000/3600 = 240m.

I did it like this.

First, while train passing thru platform in 36 sec = 15 x 36 = 540,
Second while train passing thru man in 20sec = 15 x 20 = 300,
Now, the relative length 540 - 300 = 240.

Thanks @Satheesh.

When the train enters fully from where the platform begins and at the beginning point where the man is standing it takes 20 sec so,

=54*5/18*20= 300 and that is the length of the train.

Please explain in detail that divide factor.

According to me, it should be what is the length of the platform from the point man standing for the answer to be 240. Otherwise, the answer is 540.

Please correct me if I am wrong.

@Vetri & @Sara both are correct.

Consider the man standing starting edge of platform. The train passes him in 20 sec means, we can measure the length of the train.

So, length of the train is,
(54km/3600sec)*20sec=300m.

Now train passed him,
Train =============>
Platform ########
Man $

From the above graph, 300m length train crossed him in 20 sec. Remaining 16sec train takes to cross the platform.
So,
Length of the platform is,
(300m/20sec)*16sec=240m.