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# A train crosses a bridge of length 500m in 40 seconds and a

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Manager
Joined: 14 Dec 2004
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A train crosses a bridge of length 500m in 40 seconds and a [#permalink]  05 Mar 2005, 21:35
A train crosses a bridge of length 500m in 40 seconds and a lamp post on the bridge in 15 seconds. What is the length of the train in metres?

187.5
375
300
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425
Manager
Joined: 28 Sep 2004
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Location: New York City
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Re: train size [#permalink]  05 Mar 2005, 21:56
Cloaked - Where is this question from? it doesn't sound correct, if it is would you explain ?
VP
Joined: 25 Nov 2004
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Re: train size [#permalink]  05 Mar 2005, 22:47
Director
Joined: 01 Feb 2003
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the q seems correct!

say l is the length of the train...as the speed of the train is constant

(L+500)/40 = L/15

so, L = 300 m
Senior Manager
Joined: 25 Oct 2004
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300......

let length be x metres

To cross the lamp post it takes 15 sec so speed of the train = x/15

to cross the bridge it takes 40 seconds- here the key is to add the length of the bridge and the length of the train - so speed is (x+300)/40

==>x/15=(x+300)/40

solving for x, length = 300
Manager
Joined: 28 Sep 2004
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cloaked_vessel wrote:
the OA is 300

Cloaked , where is this question from ?
Manager
Joined: 14 Dec 2004
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Thiw was an example given to me by a math tutor.
VP
Joined: 18 Nov 2004
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Kudos [?]: 20 [0], given: 0

300

Assuming speed of the train is constant

if X is the length, then from 1st statement

speed = X/15

from 2nd speed is = (500+X)/40

X/15 = (500+X)/40

X = 300
SVP
Joined: 03 Jan 2005
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Good job guys! It takes 15 seconds from the head of the train to pass a spot to the tail to pass the same spot, while it takes 40-15=25 seconds for the head to travel 500 meters. So divide 500 by five and multiply it by three you get 300.
Manager
Joined: 11 Jan 2005
Posts: 101
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(x+500)/y = 40
x/y = 15

where
x = length of the train
y = speed / second

you get
y = 20
x = 300
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