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a train crosses a bridge in 5a seconds and crosses anotherr

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Senior Manager
Joined: 07 Jul 2005
Posts: 399

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Location: Sunnyvale, CA
a train crosses a bridge in 5a seconds and crosses anotherr [#permalink]

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23 Jun 2006, 15:45
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a train crosses a bridge in 5a seconds and crosses anotherr train of

length equal to that of the bridge and moving in opp: dirn: in 4a

secs.if train overtakes the other train when moving in same directio

in 240 secs,what is value of 'a'?

I don't have answer choices but thought it might be an interesting question to solve.

Kudos [?]: 14 [0], given: 0

Director
Joined: 16 Aug 2005
Posts: 937

Kudos [?]: 31 [0], given: 0

Location: France
Re: PS : Train [#permalink]

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23 Jun 2006, 19:27
sgrover wrote:
a train crosses a bridge in 5a seconds and crosses anotherr train of length equal to that of the bridge and moving in opp: dirn: in 4a secs.if train overtakes the other train when moving in same direction in 240 secs,what is value of 'a'?

I don't have answer choices but thought it might be an interesting question to solve.

Ok here I go:
Let D be the length of the bridge and the second train

S1 = D/5a
S3 = S1 + S2 = D/4a

Therefore, speed of second train (S2) = S3 - S1 = D/20

Also, S2 - S1 = D/240
S1 = D/20 - D/240 = 11D/240
a = 48/11 .... phew!

This question was more Kaplan-like than Gmat-like. Dont even know if I made a mistake here...please review!
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Manager
Joined: 01 Jun 2006
Posts: 139

Kudos [?]: 6 [0], given: 0

Re: PS : Train [#permalink]

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23 Jun 2006, 20:13
[quote="gmatmba"][quote="sgrover"]a train crosses a bridge in 5a seconds and crosses anotherr train of length equal to that of the bridge and moving in opp: dirn: in 4a secs.if train overtakes the other train when moving in same direction in 240 secs,what is value of 'a'?

I don't have answer choices but thought it might be an interesting question to solve.[/quote]
Ok here I go:
Let D be the length of the bridge and the second train

S1 = D/5a
S3 = S1 + S2 = D/4a

Therefore, speed of second train (S2) = S3 - S1 = D/20

Also, S2 - S1 = D/240
S1 = D/20 - D/240 = 11D/240
a = 48/11 .... phew!

This question was more Kaplan-like than Gmat-like. Dont even know if I made a mistake here...please review![/quote]

S2= S3-S1= D/20a
I think we got S1 - S2= D/240
So D/5a - D/20a = D/240 => a=36

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Director
Joined: 06 May 2006
Posts: 790

Kudos [?]: 40 [0], given: 2

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24 Jun 2006, 10:21
V1 = d/5a
V1+V2=d/4a
V1-V2=d/240

=> (1/4a)-(1/240)=2/5a => a=36
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Uh uh. I know what you're thinking. "Is the answer A, B, C, D or E?" Well to tell you the truth in all this excitement I kinda lost track myself. But you've gotta ask yourself one question: "Do I feel lucky?" Well, do ya, punk?

Kudos [?]: 40 [0], given: 2

Senior Manager
Joined: 16 Apr 2006
Posts: 270

Kudos [?]: 246 [0], given: 2

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24 Jun 2006, 21:50
Speed be S1, s2
Length of bridge = L
Time Taken a, 4a

S1*5a = L = (s1+s2)*4a
(s1-s2)*240 = L

S1a = 4a*s2
S1 = 4*S2

3S2*240 = 20as2
a = 36

Kudos [?]: 246 [0], given: 2

24 Jun 2006, 21:50
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a train crosses a bridge in 5a seconds and crosses anotherr

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