Problem Solving

The time taken to cover the length of the platform is 12 seconds as 30 seconds was the time to cover the platform and its own length. it took 18 sec to cover its own length. Hence platform length = (12/3600)*72 km = 240 metres.

I broke my head, but I failed to comprehend as to why we would treat first part of the first sentence as "Platform length + Train length" and second part of the sentence as "Train length".
Thanks a ton in advance.

IMO: A
Really nice question....

smileforever41 wrote:I broke my head, but I failed to comprehend as to why we would treat first part of the first sentence as "Platform length + Train length" and second part of the sentence as "Train length".
Thanks a ton in advance.

Just imagine that a train's engine is entering in the platform, This is the moment when train start covering the platform, Now when engine exit from platform, it already has covered the length of platform, but train will leave the platform after its last wagon exit from platform.

So distance covered by engine from start to exit of platform = actual length of platform

I think it helps..

Can somebody please explain what "man standing on the platform in 18 seconds" means? I just can't understand the wording..thanks

240

Answer is A

elenaelena wrote:Can somebody please explain what "man standing on the platform in 18 seconds" means? I just can't understand the wording..thanks

U don't have to consider that as a man. You can take that as a static point that the train crosses. The action of a train crossing a static point can be described as => The front end of the train touches a static point, the entire length of the train passes by the static point till the back end of the train touch the point. So, it means that the length traveled is nothing but the train's length. Speed with which it crosses the static point is the Train's speed. Here time and speed is given with which you can find the length of the train.

Hope this Helps !

gughanbose wrote:

elenaelena wrote:Can somebody please explain what "man standing on the platform in 18 seconds" means? I just can't understand the wording..thanks

U don't have to consider that as a man. You can take that as a static point that the train crosses. The action of a train crossing a static point can be described as => The front end of the train touches a static point, the entire length of the train passes by the static point till the back end of the train touch the point. So, it means that the length traveled is nothing but the train's length. Speed with which it crosses the static point is the Train's speed. Here time and speed is given with which you can find the length of the train.

Hope this Helps !

Thanks a lot, now I have much better understanding, hope I can recognize this kind of problem on my exam.

IMO: the answer to this question should be D as if the place where the person is standing is not told in the problem,so it is required that the length of the platform should be the time taken for the train to cross the platform i.e. distance=speed *time, 72* 30=2160*1000/3600=600 metres.
Overall the question should have been what is the length of the train to arrive at 240 metres i.e. when you calculate the distance when the train crosses the man to the distance taken to cross the platform which is D=s*t,D=72*18=1296*1000/3600=360. So while calculating the length of the train we just need to subtract the two length's which is the time taken to cross the platform (-) the time taken to cross the man, 600-360=240 metres.

when a train crosses the platform it has length of platform and it's own length to cover so,
length of train + length of platform= 72 *(5/18) *30 meters--eqn(1)
length of train(when he crosses a man its only its length)
length of train= 72 * (5/18) * 18 meter --eqn(2)
so eqn(1) - eqn(2) = 240 meters

coolanubhav2002 wrote:IMO: the answer to this question should be D as if the place where the person is standing is not told in the problem,so it is required that the length of the platform should be the time taken for the train to cross the platform i.e. distance=speed *time, 72* 30=2160*1000/3600=600 metres.
Overall the question should have been what is the length of the train to arrive at 240 metres i.e. when you calculate the distance when the train crosses the man to the distance taken to cross the platform which is D=s*t,D=72*18=1296*1000/3600=360. So while calculating the length of the train we just need to subtract the two length's which is the time taken to cross the platform (-) the time taken to cross the man, 600-360=240 metres.

Dear Friend,
No need to complicate the things which are simple enough.
We are asked to calculate the length of platform. we have been told that the train speed is 72 kmph (or 20 mtr/sec).
When train crosses the platform, it covers the distance equal to its own length + platform length.
so the equation will be
Lg platform + Lg train = 20 X 30
Lg platform + Lg train = 600 At this point we only need the trains length. it can be calculated when a train will cross an object having negligible length i.e. standing man, electric pole, small tree etc. In such case train will only cover the distance equal to its own length.

Lg of Man + Lg of Train = 20 X 18
Lg of Man + Lg of Train = 360
Now we know that the length of man is negligible (can say zero) so the length of train will be 360 and that of platform will be 240.
The position of man on platform is absolutely not required here.
You can observe here that the second statement train crosses a man standing on platform is 18 seconds is given specifically for the purpose of calculation of trains length.

Regards,

Abhijit[/i]

(A) 240 meters

Prior to seeing the responses, I did not take in to account that the train was longer than the platform. Now I understand how the problem is to be worked. Is it safe to assume that the gmat will ask this type of question? One where the ENTIRE vehicle crosses an object and not just the front of the vehicle?

A it is:
30-18=12
12* 20m/s = 240 m

Does such questions appear in the GMAT exam...Anyways one has to make an assumption in mind that the man's width is negligible as compared to that of train and obviously the platform