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

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4 couples are seating at a round tables how many ways can [#permalink] New post 20 Apr 2012, 16:07
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4 couples are seating at a round tables how many ways can the 4 women sit together


source : saskschools.ca
I am trying to understand circular permutation
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Re: CIRCULAR PERMUTATION [#permalink] New post 20 Apr 2012, 23:00
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keiraria wrote:
hello

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

please explain your reasoning


best regards

source : saskschools.ca
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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Re: CIRCULAR PERMUTATION [#permalink] New post 11 Jul 2012, 21:46
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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Re: CIRCULAR PERMUTATION [#permalink] New post 12 Jul 2012, 00:39
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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!.

Questions about this concept to practice:
in-how-many-different-ways-can-4-ladies-and-4-gentlemen-be-102187.html
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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PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: 4 couples are seating at a round tables how many ways can [#permalink] New post 12 Jul 2012, 01:20
Thanks Bunuel.....
I got it and thanks for posting additional links for practice.
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Re: 4 couples are seating at a round tables how many ways can [#permalink] New post 12 Jul 2012, 20:47
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Re: 4 couples are seating at a round tables how many ways can   [#permalink] 12 Jul 2012, 20:47
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