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Imagine a parking lot with 999 cars with license plates

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AkamaiBrah
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Imagine a parking lot with 999 cars with license plates [#permalink] New post   Updated on: 13 Aug 2003, 08:14
Imagine a parking lot with 999 cars with license plates numbered from 001 to 999 and no two cars having the same license plate number. At 5pm, they all leave the lot one by one. What is the probability that the license plate numbers of the first four cars to leave are in increasing order of magintude?



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Originally posted by AkamaiBrah on 12 Aug 2003, 07:45.
Last edited by AkamaiBrah on 13 Aug 2003, 08:14, edited 1 time in total.
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Re: Imagine a parking lot with 999 cars with license plates [#permalink] New post   12 Aug 2003, 07:58
AkamaiBrah wrote:
Imagine a parking lot with 999 cars with license plates numbered from 001 to 999 and no two cars having the same license plate number. At 5pm, they all leave the lot one by one. What is the probability that the license plate numbers of the first four cars to leave are in increasing order of magintude?


The # of the first car can be anywhere, but the AVERAGE of all of the places it could be is right in the middle, so the average chances of the second car being greater than this value is 1/2.

Now the third car has to be above the second, and the chance of this is 1/4. And finally, the fourth must be still above that so 1/8.

1/2 * 1/4 * 1/8 = 1/64
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Re: Imagine a parking lot with 999 cars with license plates [#permalink] New post   12 Aug 2003, 08:06
Is it 999C4 * (1/2)^4 ?
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Re: Imagine a parking lot with 999 cars with license plates [#permalink] New post   12 Aug 2003, 08:15
kpadma wrote:
Is it 999C4 * (1/2)^4 ?


Kpadma, your answer equals way over 1, with 1 being a certain probability. I don't think that passes a sanity check.
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Re: Imagine a parking lot with 999 cars with license plates [#permalink] New post   12 Aug 2003, 11:22
AB,

Here I come charging again.

P = (997)^2 / (8 * 999 * 998 ) = (approx) 1/8

I think I may be close to the answer, but not sure it the correct answer.
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Re: Imagine a parking lot with 999 cars with license plates [#permalink] New post   12 Aug 2003, 12:16
kpadma wrote:
I think I may be close to the answer, but not sure it the correct answer.



sounds like yogi berra.
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Re: Imagine a parking lot with 999 cars with license plates [#permalink] New post   13 Aug 2003, 08:14
AkamaiBrah wrote:
kpadma wrote:
I think I may be close to the answer, but not sure it the correct answer.



sounds like yogi berra.


Any four cars have an equal chance of leaving the lot first, so we can concentrate on just one specific bunch of four cars. (Whether there are 999 or just 4 cars in the lot is irrelevant). For a given set of four cars, they can leave the lot in 4! or 24 ways, only one of which the license plate numbers will be in increasing order. Hence, the answer is 1/24.
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Re: Imagine a parking lot with 999 cars with license plates [#permalink] New post   13 Aug 2003, 09:14
A nice trick! Remember a question with six letters to be distributed among six envelopes? What is the probability of having them all distributed correctly? 1/6! Again, there is the only right case.



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Re: Imagine a parking lot with 999 cars with license plates [#permalink]
13 Aug 2003, 09:14
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