Aptitude - Problems on Trains - Discussion

I think there should be clarified in the question. That the man is exactly standing the place as the train is long.

Then only we can take the train length according to given solution.

Otherwise, the solution is wrong.

According to my understanding,

1- In 36s train passes the total length which is unknown.
2- In 20s train passes the man which will be our reference point to calculate the speed.

So, from formula: Length or Distance = Time * Speed.

Length of train = 20s * 54(1000/60 * 60).
Length of train = 300 m.

Total length (Platform & train) = 36s * 54(1000/60 * 60).
Total length (Platform & train) = 540m.

Platform length = Total length = Train length = 540 -300 = 240m.

The length of the train = the time * speed
= 15 * 20 seconds taken by it to pass the man as the man is standing at one point.
= 300m

The length of the platform = remaining time * speed.
= (36 - 20) * 15.
= 16 * 15.
= 240.

36 - 20 = 16,
54 * 5/18 = 15,
16 * 15 = 240.

Why we multiply 15*20?

@ALL.

Logically, the length of the train should be more or less equal to the length of the platform else it will be troublesome for the passengers to cover that distance. And the solution assumes that the man is standing at the beginning of the platform.

For finding length of the train we must consider a reference point (man).

But for finding the distance covered why do we need the length of the train.

Simply we can do DISTANCE COVERED = SPEED X TIME.

Can anyone explain it, whether this is correct method are wrong?

@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.