Aptitude - Problems on Trains - Discussion

@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!

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.

I think there are two kind of speeds are involved there. One is the speed by which it crosses the pole, second is the speed by which it crosses the platform. Once we calculate the speed Vp of train by which it crosses the platform (300/39 m/s), signal crossing speed Vs (300/18 m/s).
V = Vs-Vp = 8.97 m/s.
t = 39s.
S = 350meter is the answer.

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.

Speed 50/3 is correct. Because platform has a length. But pole doesnt. Its only a point that the train crossed. So the distance covered when it crosses the pole is just the train's length.

That is 300 metres. So the train moves at a speed of 300 metres per 18 seconds. Speed is 300/18. Hence 50/3.

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.

According to me;

300m long train cross-platform in 39 sec.
Cross pole in 18 sec (39-18= 21sec).

(Cross-Platform means platform +train)
(Cross Pole means only train length).

Extra time it takes 21 sec.
"18 sec ------ 300m.
21sec -------???? M.
21/18 * 300 = 350m.

@All.

Usually take 300+x and 300+y for platform and signal respectively but if we go by this there isn't enough data they have taken the pole as the stationery object as the beginning that is y it is so confusing they haven't mentioned something like that and did it.

@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.