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A train of length L is traveling at a constant velocity and passes a

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A train of length L is traveling at a constant velocity and passes a [#permalink] New post 02 Jul 2010, 22:35
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A train of length L is traveling at a constant velocity and passes a pole in t seconds. If the same train travelling at the same velocity passes a platform in 3t seconds then what is the length of the platform:

A) 0.5 L
B) L
C) 1.5 L
D) 2 L
E) 3 L
[Reveal] Spoiler: OA
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 03 Jul 2010, 00:33
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A train of length L is traveling at a constant velocity and passes a pole in t seconds. If the same train travelling at the same velocity passes a platform in 3t seconds then what is the length of the platform:
A) 0.5 L
B) L
C) 1.5 L
D) 2 L
E) 3 L


Sol:
Length of train=Distance=L
time to cross the pole=time=t
so speed=L/t

new time=3t. so distance travelled= 3t*L/t = 3L

the distance is equal to train length L+platform length = 3L
so platform is 2L. ans is D.
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 03 Jul 2010, 11:08
appy001 wrote:
A train of length L is traveling at a constant velocity and passes a pole in t seconds. If the same train travelling at the same velocity passes a platform in 3t seconds then what is the length of the platform:
A) 0.5 L
B) L
C) 1.5 L
D) 2 L
E) 3 L


Sol:
Length of train=Distance=L
time to cross the pole=time=t
so speed=L/t

new time=3t. so distance travelled= 3t*L/t = 3L

the distance is equal to train length L+platform length = 3L
so platform is 2L. ans is D.


Ahh - very sneaky question! It's easy to forget that the entire train has to pass by the platform, so it travels the extra L distance.

Nicely done!
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 31 Oct 2014, 22:47
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 01 Nov 2014, 04:40
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shamikba wrote:
A train of length L is traveling at a constant velocity and passes a pole in t seconds. If the same train travelling at the same velocity passes a platform in 3t seconds then what is the length of the platform:
A) 0.5 L
B) L
C) 1.5 L
D) 2 L
E) 3 L


Speed = L/t = (L+P)/3t. So P = 2L
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 01 Nov 2014, 07:40
The distance the train is traveling isn't just L, it's L plus the length of the pole. This question assumes that the length of the pole zero.
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 20 Dec 2014, 20:38
Length of pole is assumed 0. From case 1 we get the speed of the train =L/t
In case 2 we need distance =speed x time =L/t x 3t=3L, but this 3L is the total distance inclusive of the length of the train which is L. so length of the platform =3L-L=2L.
Hope this explanation helps.
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 21 Dec 2014, 18:41
Expert's post
Hi All,

This question can be solved by TESTing VALUES.

We're told that a train has a length of L and passes a pole in T seconds.

Let's TEST:
L = 3 (feet)
T = 2 (seconds)

Since the pole is NOT given a length, we have to assume that that value is negligible (and does NOT factor into the calculations).

So, the entire length of the train passes the pole in 2 seconds. Thus, it's traveling 3 feet every 2 seconds.

Next, we're told that this train passes a platform in 3T seconds. This means that the train travels 3(2) = 6 seconds and travels 3(3) = 9 feet.

Those 9 feet are made up of the train (3 feet) and the platform (9 - 3 = 6 feet).

The question asks us for the length of the PLATFORM. Using these VALUES, we know that the PLATFORM = 6 when L = 3.

Final Answer:
[Reveal] Spoiler:
D


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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 24 Jan 2016, 08:01
Hello from the GMAT Club BumpBot!

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A train of length L is traveling at a constant velocity and passes a [#permalink] New post 24 Jan 2016, 14:06
let r=rate
let p=length of platform
L+p=(3t)(r)=3rt
(L+p)/3=rt
L=rt
(L+p)/3=L
L+p=3L
p=2L
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Re: A train of length L is traveling at a constant velocity and passes a [#permalink] New post 24 Jan 2016, 21:46
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shamikba wrote:
A train of length L is traveling at a constant velocity and passes a pole in t seconds. If the same train travelling at the same velocity passes a platform in 3t seconds then what is the length of the platform:

A) 0.5 L
B) L
C) 1.5 L
D) 2 L
E) 3 L



Look at the question logically:

"A train of length L is traveling at a constant velocity and passes a pole in t seconds"
To cross a pole (which can be considered a point), why does the train take t secs? Because the whole length of the train has to cross it.

" If the same train travelling at the same velocity passes a platform in 3t seconds"
Why does the train take extra 2t secs now? Because of the length of the platform. So the length of the platform must be 2L.

Answer (D)
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Re: A train of length L is traveling at a constant velocity and passes a   [#permalink] 24 Jan 2016, 21:46
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