Aptitude - Problems on Trains - Discussion

Why should I take 300/18 ? Why shouldn't I take 300/39 ?

Please give explanation.

HI..friends...As per my knowledge....39sec is the time that crosses the platform. whereas 18sec is the time that crosses the signal. Here 300/18 is the speed.i.e the relative speed.But 300/39 is the total time that taken to cross the platform. By this 39sec we have to find out the total length of the platform by using the total time taken to cross the platform.Thats why u have to take 300/18 instead of 300/39.

@Atique

We cannot take 300/39 to find the speed of the train.

Because,

Case-1:

39 seconds is the time taken by train to cross a platform. With this information, to find the speed of the train we need length-of-the-train + length-of-the-platform. But, here the length of the platform not given, we have to find it actually.

Case-2:

18 seconds is the time taken by train to cross a single pole. With this information, to find the speed of the train we need length-of-the-train only (length-of-the-pole is negligible, because it is a single point). So with this 18 sec and 300m we can find the speed of the train.

How to find the speed of the train?

Speed = Distance/Time

Given Info:
Distance = 300 m.
Time = 18 sec (to cross a single pole).

Therefore,

Speed = 300m/18sec = 50/3 m/sec.

Take a look at this page:
http://www.indiabix.com/aptitude/problems-on-trains/discussion-454

Hope this will help you. Have a nice day!

Friends .

Shortcut:

speed =300/18=50/3m/sec
extra time taken to cross the platform=39-18=21
50/3*21=350m the length of platform

@Sundar
Question specifies 300m long train.
So the length of the train is 300m.

The link which u gave there we were supposed to find the length of the train so we followed that method.

But,here the length of the train is already given as 300m.

Length of the train is 330m.

So if the train cross the pole it take 18sec.

So we find speed of it.

Speed=300/18=50/3.

Total time is 39sec.

But we want only length of platform.

i.e., 39-18=21.

Finally length of it =50/3*21.

=350m.

For the case 1,where you told " to find the speed of the train we need length-of-the-train + length-of-the-platform. But, here the length of the platform not given, we have to find it actually.", I can assume that it will b x..i.e

(x+300)/18 = 100/13.

I can't understand why you find out 300/18. Please sir elaborate it.

The reason this question doesn't have an answer to me is because of the wording if the train has to cross the platform shouldn't the length of the train Be a factor? Because the train has to travel more meters than the platform to actually cross it and the question don't seem to account for that.

Hey why should we want to use x+300 here?

Hai dear sir, I am very confuse please help me explain that how we will measure the speed in detail with example?