Problem Solving

IMO A

Here is my working

Train is travelling @ 72km / hour, this is equivalent to
- 1.2km / minute, which is equivalent to
- 1,200m / minute (as there are 1000m per km), which in turn equates to
- 20m / second (divide 1200m by 60s)

It takes the train 12 seconds to cross the entire platform (i.e., 30s - 18s)

Therefore, length of platform in m = 12s x 20m = 240m = ANSWER A

To "cross" the platform, the entire train would have to have passed the platform.

Imagine the head the of train starting at point A of the platform, then reaching point B of the platform. At point B, the rest of the train, all 360 meters of it, have yet to reach point B. We cannot say that the train's "crossed" the platform until we wait for the rest of the train to cross point B. This is why you have to subtract the length of the train (360) from the length of the total distance (600) traveled.

I am glad that everyone here on this forum is working together for a common goal . Thanks for taking time and making all possible diagrams.

Anyhow, many people have answered this problem quite interestingly.

I solved the problem in this way.

Step 1:

Find out the speed of the train in m/s.

72kmph = (72*1000)/(60*60) = 20 m/s

Step 2:

Find the length of the train.

The reference of the man in the question is a clue to find out the length of the train. A man is like a point on the platform. The train takes 18 seconds to cross the point (man) i.e

Length of the train = 18 Secs * 20 m/s = 360 m


Step 3:

Find the length of the platform.

It takes the train 30 seconds to cross the platform.

So, the train covers a distance of (30 Secs * 20 m/s) = 600 m

600 m is the sum of the lengths of the train and the platform.

(Note: We are considering length of the train in 600 m because the time is calculated from the instant the train starts crossing the platform to the instant the train exits the platfrom)

As we have calculated in step 2, the length of the train is 360 m

So, the length of the platform = 600 m - 360 m = 240 m


So, the answer is (A).

CLARIFICATION FOR THOSE WHO ARE SUPPORTING 600M

There is one basic logic in this question, thats why we are given time to pass man and platform.

Just imagine that Train has two ends and length of it. While passing a man, Train is actually covering only it's own length. so we can find easily length of train (as calculated by many members) i.e. 360m.

In measuring length of Platform, the time given 30sec is basically a time when Train's head just passes start point of Platform and time finishes when trains tail passes other end of platform. So it actually covers "Trains own length + platform length". so total length of Platform is 600m - 360m = 240m.

speed of the train = 72000/3600 = 20 meters / sec.

So lenght of the train = 18 * 20 = 360 meteres

Time taken to cross the platform = 30

Total distance covered in 30 sec = 20 * 30 = 600 m

lenght of platform = 600 - 360 = 240 meters

A is correct.


The length of train is: T___N
The length of platform is : P_______M

At beginning:
N travel from P to M = length of platform
However, in order to train cross platform <=> T at M =the distance between N and T = length of train.

So, train cross platform = length of platform + length of train.

Man standing on platform at which point is not important. It is only a element to solve the length of train.
In this question, understanding word "cross" is impotant. We need visualize the question..

Length ofplatform + length of train = 600 m
length of train = = 360 meters

length of platform = 240 m

I think this is a poorly worded problem any way you look at it. Ambiguous and open to interpretation. In my opinion, the guy must have died after the train "crossed him" (what is that supposed to mean?--How does one get crossed by a train and in the same vein; how does a train cross a platform?). Not trying to be funny or cantankerous, but I found this question rather unhelpful. The GMAT questions I have become accustomed to are far more concise and solid. I hope I have not annoyed anyone but that's just my opinion and I am but one fallible person. Thank you for otherwise good discussions and questions.

Guys, Rahul is correct. The position of the standing man isnt important here. It's merely given so that we can figure out the length of the train.

Sketch a platform quickly and try to visualize how a train covers the platform. Its Engine first reaches the start of the platform and then reaches the other end of the platform (irrespective of where the last bogey is). This gives us the length of the platform. The 'whole' train covers the platform when the last bogey has just crossed the platform (irrespective of where the engine is). So, if the train covers the platform in 30 seconds, the engine has to travel the length of the platform + length of the train in 30 seconds. Upon calculating, we get length 600m. This length is the length of the platform + length of the train.

Length of the train can be figured out from the other data given. Width of the man is considered negligible (Must be a very very lean guy )

I hope my explanation is good enough.

i thought the answer to be option D but its wrong.i solve this way by converting the time to hour and then multiplying it with speed to find the distance 600 but i am not able to understand further steps

Can someone please provide me with a step by step process of how the answer was derived?

Whoever wrote this question obviously did not study for the verbal portion of the GMAT.

This question has too many unanswered parts which need assumptions.

1) 18 seconds to cross (Just the man?)
2) At what speed was the man moving? Because if he was moving that needs to be factored in relative velocity?
3) What does crossing the platform mean? with respect to itself or with respect to the man? First, case you would get 600m as the answer, second case answer is 240.
4) Does cross the person mean the front end or the rear end of the train?
5) Does cross the platform mean the front end or the read end of the train?
PS: When somebody crosses the finish line do we say its the REAR END?? NO, so why does the train length have to be factored in this problem?


SUMMARY: BAD QUESTION WHICH IS TOTALLY NOT GMAT STYLED!