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Trains puzzle

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Trains puzzle [#permalink]

08 Feb 2011, 06:10

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Two stations connected by railroad. Trains start every hour from each station to the other, at the same time. A Train takes 5 hours to reach the other station. How many trains will a train meet during its journey?

I think the answer should be:

[Reveal] Spoiler:

But no OA is provided.

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Re: Trains puzzle [#permalink]

08 Feb 2011, 22:58

Assuming that all trains travel at the same speed,

So a train from Station A will meet a train from station B in 2.5 hrs

Within that span of time, two trains would have departed from station B.

Our train from station A would meet these two trains also.

So overall, the train meets 3 other trains

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Re: Trains puzzle [#permalink]

08 Feb 2011, 23:35

Let's use 2400 hrs time notation.

A@n to symbolize a train's departure at "n*100" hrs from A. e.g. A@8 is train starting from A at 800 hrs
B@n to symbolize a train's departure at "n*100" hrs from B. e.g. B@10 is train starting from B at 1000 hrs

A@8 won't meet the train B@3 because B@3 already arrived at A when A@8 left the station. However, A@8 will meet every train after B@3 viz.
B@4,B@5,B@6,B@7,B@8,B@9,B@10,B@11,B@12. A total of 9 trains.

A@8 won't meet B@13 because it already reached station B at 1300 hrs, 5 hours after 800hrs.

Ans: 9
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Director

Status: -=GMAT Jedi=-

Joined: 04 Jan 2011

Posts: 745

Location: Kochi, India

Schools: ISB

WE 1: Engineer - Larsen & Toubro, ECC Division

WE 2: Faculty - T.I.M.E.

Followers: 27

Kudos [?]: 103 [0], given: 66

Re: Trains puzzle [#permalink]

08 Feb 2011, 23:37

@Fluke
Isn't the question about 'a' train?

So the answer can never be more than 4
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Director

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Re: Trains puzzle [#permalink]

09 Feb 2011, 05:25

Fluke, you are right. Thanks. I got it.

Re: Trains puzzle [#permalink] 09 Feb 2011, 05:25

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