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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]

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New post 02 Jul 2010, 23:35
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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
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Re: A train of length L is traveling at a constant velocity and passes a  [#permalink]

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New post 03 Jul 2010, 01:33
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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]

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New post 03 Jul 2010, 12:08
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1
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]

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New post 01 Nov 2014, 05:40
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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]

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New post 01 Nov 2014, 08:40
1
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]

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New post 20 Dec 2014, 21:38
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]

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New post 21 Dec 2014, 19:41
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.

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A train of length L is traveling at a constant velocity and passes a  [#permalink]

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New post 24 Jan 2016, 15:06
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]

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New post 24 Jan 2016, 22:46
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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]

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New post 16 Aug 2017, 11:56
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


It's a very simple question..
So, Train of length L passes a pole in t seconds. Velocity of pole v= L/t

Now this train of length L passes platform of say length p passes in 3t seconds .
So, (L+p)= v (3t) = L/t(3t) 3L
-> p = 2L

Answer D
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Re: A train of length L is traveling at a constant velocity and passes a  [#permalink]

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New post 16 Aug 2017, 11:59
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


We can also analyse it and solve it in a very short span of time as VeritasPrepKarishma suggested...

Train of length L passes a pole in t seconds i.e. it covers L in t seconds.
So, in 3t seconds it covers 3L distance, out of which L is length of train and 2L is length of platform... very simple, isn't it? :)
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Re: A train of length L is traveling at a constant velocity and passes a  [#permalink]

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New post 07 Jul 2018, 18:33
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


We can let p = the length of the platform in meters and r = the rate the train is traveling.

When we say the train crosses a platform in 3t seconds, it really means it takes 3t seconds for the nose of the train to enter one end of the platform and the tail of the train to exit the other end of the platform. Thus, in 3t seconds, not only does the train travel the entire length of the platform but also it travels its body length L. Thus, we have (using time x rate = distance formula):

3t * r = p + L

We are also given that the train crosses a pole (notice that the pole has a negligible width) in t seconds. So when the train crosses the pole, it only travels its body length in t seconds. Thus we have:

t * r = L

Subtracting these two equations, we have:

2tr = p

Since L = tr, and p = 2tr, then p = 2L.

Answer: D
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Re: A train of length L is traveling at a constant velocity and passes a &nbs [#permalink] 07 Jul 2018, 18:33
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