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# 4 couples are seating at a round tables how many ways can

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Intern
Joined: 05 Apr 2012
Posts: 46

Kudos [?]: 37 [0], given: 12

4 couples are seating at a round tables how many ways can [#permalink]

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20 Apr 2012, 17:07
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Question Stats:

67% (00:40) correct 33% (01:55) wrong based on 9 sessions

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4 couples are seating at a round tables how many ways can the 4 women sit together

I am trying to understand circular permutation

Kudos [?]: 37 [0], given: 12

Math Expert
Joined: 02 Sep 2009
Posts: 42356

Kudos [?]: 133203 [0], given: 12439

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21 Apr 2012, 00:00
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keiraria wrote:
hello

4 couples are seating at a round tables
how many ways can the 4 women sit together

best regards

I am trying to understand circular permutation

4 men can be arranged around a table in (4-1)!=3! ways;

Now, we can place 4 women in 4 slots between any two men and arrange these women in this slot in 4! ways.

So, total # of ways is 3!*4*4!=4!*4!

Another way: if 4 women sit together around a table then 4 men also sit together, so total # of arrangements is 4!*4!.

Check this for circular arrangements: math-combinatorics-87345.html

Hope it helps.
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Intern
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11 Jul 2012, 22:46
Can we put all the women in 1 group.
Total will be : 4 men + 1 group of women = 5
To arrange these 5, we will have 5! ways.
Again to group all the women we will have = 4! ways.
Then total ways = 5! X 4!

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Math Expert
Joined: 02 Sep 2009
Posts: 42356

Kudos [?]: 133203 [0], given: 12439

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12 Jul 2012, 01:39
manasrastogi wrote:
Can we put all the women in 1 group.
Total will be : 4 men + 1 group of women = 5
To arrange these 5, we will have 5! ways.
Again to group all the women we will have = 4! ways.
Then total ways = 5! X 4!

The red part is not correct.

Notice that we have circular not linear arrangement.

The difference between placement in a row and that in a circle is following: if we shift all object by one position, we will get different arrangement in a row but the same relative arrangement in a circle. So, for the number of circular arrangements of n objects we have: $$\frac{n!}{n}=(n-1)!$$.

Hence 4 men and 1 group of four women (total of 5 units) can be arranged around a circular table in (5-1)!=4! ways. Next, these 4 women within their unit can be arranged in 4! ways, which gives total equal to 4!*4!.

at-a-party-5-people-are-to-be-seated-around-a-circular-104101.html
seven-family-members-are-seated-around-their-circular-dinner-102184.html
seven-men-and-seven-women-have-to-sit-around-a-circular-11473.html
a-group-of-8-friends-sit-together-in-a-circle-alice-betty-106928.html
seven-men-and-five-women-have-to-sit-around-a-circular-table-98185.html
a-group-of-8-friends-sit-together-in-a-circle-alice-betty-106928.html
find-the-number-of-ways-in-which-four-men-two-women-and-a-106919.html

Hope it helps.
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Kudos [?]: 133203 [0], given: 12439

Intern
Joined: 21 Jun 2012
Posts: 2

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Re: 4 couples are seating at a round tables how many ways can [#permalink]

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12 Jul 2012, 02:20
Thanks Bunuel.....

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Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
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Location: Pune, India
Re: 4 couples are seating at a round tables how many ways can [#permalink]

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12 Jul 2012, 21:47

Kudos [?]: 17877 [0], given: 235

Re: 4 couples are seating at a round tables how many ways can   [#permalink] 12 Jul 2012, 21:47
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