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circular permutation

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Junior | Next Rank: 30 Posts
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by ohwell » Sat Nov 22, 2008 8:59 pm
Find the number of ways in which five persons can sit around a circular table, when two of the persons insist on sitting next to each other?

I went as far as drawing all the combinations and I reached at 10, but the answer is aparantly 12. Can anyone explain?
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Re: circular permutation

by logitech » Sat Nov 22, 2008 9:48 pm
ohwell wrote:Find the number of ways in which five persons can sit around a circular table, when two of the persons insist on sitting next to each other?

I went as far as drawing all the combinations and I reached at 10, but the answer is aparantly 12. Can anyone explain?
You should use (n-1)! to find the circular permutation.

The reason is you can sort N people in n! ways but for the n times you will have same combination with two different starters since they are in circle:

So : n!/n = (n-1)!

Since two people wants to sit next to eachother, the total group will act like 4 people

12 3 4 5

(4-1)! = 6 ways

but since they can also switch places:

21 3 4 5

6 x 2 = 12 ways.
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by ohwell » Sat Nov 22, 2008 10:00 pm
thx!
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