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13 persons take their places at a round table. What is teh

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13 persons take their places at a round table. What is teh [#permalink] New post   23 Jun 2006, 16:47
13 persons take their places at a round table. What is teh probability that 2 particular persons do not sit together?



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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   23 Jun 2006, 21:00
I think the prob is 10/12=83.33%
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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   23 Jun 2006, 23:03
I think its 1-(11!/13!)=1-(2/12)=5/6
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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   23 Jun 2006, 23:23
= 1 - 2(11!) / 12!
= 5/6


helg wrote:
I think its 1 - (11!/13!) = 1 - (2/12) = 5/6


how "11!/13! = 2/12 ?"
11!/13! = 1/(12x13)
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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   24 Jun 2006, 03:20
it is much easier way to find the answer
if we have A,B and A has got a place. So we only have 10 choices for B in the left 12 places.
It means that the prob is 10/12=5/6
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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   24 Jun 2006, 09:30
quangviet512 wrote:
it is much easier way to find the answer
if we have A,B and A has got a place. So we only have 10 choices for B in the left 12 places.
It means that the prob is 10/12=5/6


beautiful :!:

Got the same answer, but used the factorial method...
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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   24 Jun 2006, 10:14
quangviet512 says

Quote:
it is much easier way to find the answer
if we have A,B and A has got a place. So we only have 10 choices for B in the left 12 places.
It means that the prob is 10/12=5/6


Out of 13 places if A takes first place then there should 11 places available for B. so prob should be 11/12

I think I am missing something.

quangviet512 Can you explain it further !!

Thanks in advance
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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   24 Jun 2006, 10:20
Nope. B has 10 places only. Out of 13, one place is taken by A, and B can't sit either to the left, or to the right of A. Hence 10.
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Re: 13 persons take their places at a round table. What is teh [#permalink] New post   25 Jun 2006, 01:26
Professor wrote:
= 1 - 2(11!) / 12!
= 5/6


helg wrote:
I think its 1 - (11!/13!) = 1 - (2/12) = 5/6


how "11!/13! = 2/12 ?"
11!/13! = 1/(12x13)


yep, you are right.
don't know how I could get this result :(



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Re: 13 persons take their places at a round table. What is teh [#permalink]
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