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SPEED TIME

by francoisph » Mon Jun 21, 2010 12:26 pm
A train traveling at 72 kmph crosses a platform in 30 seconds and a man standing on the platform in 18 seconds. What is the length of the platform in meters?
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by tpr-becky » Mon Jun 21, 2010 1:06 pm
Is this the entire question? I don't understand what it means by crosses a platform and a man standing on the platform. IN general this is a D=R*T question and I believe a conversion question - becuause you have to change for kilometers to meters and from hours to seconds.

D=72kmph(30sec)(1/60(sec))(1/60min)(1000m/km)= 650 meters.
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by francoisph » Mon Jun 21, 2010 1:12 pm
Yeah

The correct choice is (A) and the correct answer is 240 meters.

I didnt get the answer 240 meters
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by amising6 » Mon Jun 21, 2010 7:02 pm
francoisph wrote:A train traveling at 72 kmph crosses a platform in 30 seconds and a man standing on the platform in 18 seconds. What is the length of the platform in meters?
hey
there are two concept
when train crosses a man or post
it will have to cover its own length so distance travelled will be equal to the length of train
when train crosses a platform or bridge
it will have to cover its own length +length of bridge
so distance travelled will be equal to length(bridge)+length(train)

now coming to question:
speed =72kmph i.e 72*5/18=20 m/sec
time=30 sec
so distance travelled by train=speed *time=20*30=600 m =(length(platform)+length(train))
600 m =(length(platform)+length(train))

now second part
a man standing on the platform in 18 seconds. What is the length of the platform in meters?[/quote]
speed of train 72kmph=72*5/18=20 m/sec
time =18 sec
distance=20*18=360 m and we know that when train crosses a man it covers its own length
so length of platform=600-360=240 metres
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by BastiG » Mon Jun 21, 2010 7:20 pm
You could save some time if you just multiply 600 with 2/5 that equals 240.
The train has the whole distance the same rate (speed) and 18 seconds is 3/5 of 30 seconds. But you want to know the distance between 18 and 30 seconds and this is 2/5.
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by Testluv » Mon Jun 21, 2010 7:37 pm
I'm inclined to agree with Becky: this question is either incomplete or ambiguous, or ambiguous because it is inomplete, etc.
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by francoisph » Tue Jun 22, 2010 12:10 pm
Ambigious agreed! but the OA is below

When the train crosses a man standing on a platform, the distance covered by the train is equal to the length of the train.

However, when the same train crosses a platform, the distance covered by the train is equal to the length of the train plus the length of the platform.

The extra time that the train takes when crossing the platform is on account of the extra distance that it has to cover = length of the platform.

Therefore, length of the platform = speed of train * extra time taken to cross the platform

Length of platform = 72 kmph * 12 seconds

Converting 72 kmph into m/sec, we get 72 kmph *5/12= = 20 m/sec

Therefore, length of the platform = 20 * 12 = 240 meters.
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by tpr-becky » Tue Jun 22, 2010 4:43 pm
While I now understand what the question was getting at I do not think it is always productive to work with questions that don't have answer choices - it isnt' a realistic representation of what will be on the exam and often creates more difficulties than it solves.
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by francoisph » Wed Jun 23, 2010 3:51 am
that's why gmat is a nightmare for no english people!
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by tpr-becky » Wed Jun 23, 2010 7:57 am
I do understand how difficult the exam in for non- native english speaking people. I have taught quite a few ESL students in the past years and you certainly have a difficult job ahead of you. I am always amazed at the determination of people to achieve their dreams and think you are very brave.
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by Testluv » Wed Jun 23, 2010 9:21 am
tpr-becky wrote:I do understand how difficult the exam in for non- native english speaking people. I have taught quite a few ESL students in the past years and you certainly have a difficult job ahead of you. I am always amazed at the determination of people to achieve their dreams and think you are very brave.
Awesome words of encouragement!
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by outreach » Wed Jun 23, 2010 9:50 am
1km/hr=5/18 m/s
72km/hr=S

s=20m/s

when the train crosses a man in 18 seconds(mans length considered invisible)
length of train=distance covered in 18 seconds
=speed * time=20*18=360m

distance covered in 30 sec=length of platform+length of train
length of platform= distance covered in 30 sec - length of train
= 30*20 - 360=240m
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by francoisph » Wed Jun 23, 2010 9:56 am
It's very hard to do Gmat at 36 years old o:)

I dont know how do they get 730 score at 35 or 40 years old?
because all maths concepts are so far ................
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