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

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Manager
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A train crosses a bridge of length 500m in 40 seconds and a [#permalink]

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05 Mar 2005, 22:35
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

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
250
425
Manager
Joined: 28 Sep 2004
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05 Mar 2005, 22:56
Cloaked - Where is this question from? it doesn't sound correct, if it is would you explain ?
VP
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05 Mar 2005, 23:47
Director
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06 Mar 2005, 01:20
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
Manager
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06 Mar 2005, 15:44
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
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06 Mar 2005, 16:46
cloaked_vessel wrote:
the OA is 300

Cloaked , where is this question from ?
Manager
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06 Mar 2005, 16:54
Thiw was an example given to me by a math tutor.
VP
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06 Mar 2005, 18:23
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
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06 Mar 2005, 21:28
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
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06 Mar 2005, 22:58
(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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