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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
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] 21 Dec 2014, 18:41
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