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All - I thought it would be a good idea to put together [#permalink]
05 Jan 2005, 14:57

1

This post received KUDOS

toddmartin wrote:

Can you explain why the ways the men can be seated is 2! and not 3!

Since I cannot attach any figures, refer to this URLfor explanation of circular permuations. This is my explanation...

1-2-3-4-5-6-1 (assume this to be a closed circle)

Lets assume the position of the first man to be "fixed" at 1, then there are two remaining positions for the remaining two men: 3rd and 5th seat.
Number of ways of seating a man in 3rd seat = 2
As only one man is left, number of ways of seating a man in 5th seat is 1.
Number of ways that three men can be seated first at the round table = 1*2*1=2

There are three remaining seats 2,4, and 6.
Number of options for 2 = 3
Number of options for 4 = 2 (only 2 women are left)
Number of options for 6 = 1 (only 1 woman left)
i.e. The three women can be seated in 3! ways = 6

Re: In how many ways can 3 men and 3 women be seated around a [#permalink]
09 Jul 2015, 02:30

1

This post received KUDOS

Expert's post

iPen wrote:

The answer GMAT wants is 12. You fix one guy to a seat and alternate man-woman. Now, the other two guys can be arranged in only two different ways with respect to that one guy you fixed in one seat. So, that's 2! ways. The women can be arranged in 3! ways with respect to the one guy. So, it's 2!*3!.

However

If each chair itself is unique, then shouldn't the answer be 72? The question assumes that the seats themselves don't matter, and that only the relative arrangement of people matters. So, if the seats actually mattered, say, seat A is a terrible chair that's about to fall apart, seat B is an Eames lounge chair, seat C is a massage chair, seat D is closest to the window, seat E is closest to the bathroom, and seat F is next to a different loud table, then such a scenario does exist, and the answer would need to be 72.

So, calculate 12 as you did above from one fixed guy in seat A, then move the guy you fixed to seat B, then C, then D, etc. That's 72 combinations, since each seat with respect to one guy has 12 arrangements and there are six seats total (12*6). Another way to do it is to think of each seat as M1-W2-M3-W4-M5-W6 (man seat#; woman seat#) around a table and multiply the combination, so after simplyfing it's 3*3*2*2*1*1 = 36. Those are only for the odd numbered seats for men, and even numbered seats for women, so the answer needs to be double that, giving us 72.

If 12 and 72 were both answer choices, I'd pick 12 but with serious protest. GMAT may ask for unique orders of people only, so the same order but in different chairs may be disqualified in GMAT's eyes. But, the GMAT doesn't specify that they want only unique orders of people - is it somehow implied in the question? The question itself is insufficient to draw a conclusion, esp. if 12 and 72 are both answer choices. Any thoughts?

You are absolutely correct about both your calculations however there here is a concrete point to know about GMAT

You don't consider Chairs different until GMAT clearly mentions it and No mention of the same would be considered that the arrangement takes into account only the positions at which people sit and not the chairs on which people sit _________________

In a round table the first person can be selected in 6 ways (any men or women can be selected), next one can be selected in 3 ways (i.e. opposite gender of the person selected in the first chair), next set can be taken by 2 remaining from an opposite gender from the 2nd chair, so 2 ways, 4th seat again can be filled by 2 remaining from opposite gender from the 3rd chair, last 2 chairs can be filled 1 way each. So no of ways:

Please draw a circle and mark 6 points on it. 3 x and 3 . 1)
a) split into 3 men and 3 women.
b) take men first -> first person can be seated in any of the available seats so # of ways -> 6
c) 2 men are remaining and only 2 seats are available. Since one man is fixed, adjacent positions cannot be occupied by men, also seat exactly opposite to first man cannot be occupied by any man. Thus we have only 2 places.
d) # of ways 2nd man can sit -> 2
e) last man has only one way.
f) 3 women are remaining and 3 places. 3 out of six have been occupied by 3 men.
g) First women can be placed in 3 ways
h] second women in 2 way and
i] third women in 1 way.

Three men can be seated first at the round table in 2! = 2 ways.
Then the three women can be seated in 3 gaps in 3! = 6 ways.
Hence the required number of ways = 2 x 6 = 12

I cannot give you an answer in terms of a equation. What I can do is explain it in a diagram. So if you draw a circle and draw six lines aorund that circle representing the six places where the people sit and then put man 1(m1), m2 and m3 in their place and the women between them. If you rotate the men (3 options) and you rotate the women (3 options) around the table you get a total 6 options. If you then switch the men and women and do the same you get another 6 and add these together you get 12 posibilities. I know it is crude but it worked for me. _________________

Three men can be seated first at the round table in 2! = 2 ways. Then the three women can be seated in 3 gaps in 3! = 6 ways. Hence the required number of ways = 2 x 6 = 12

Can you explain why the ways the men can be seated is 2! and not 3!

In a round table the first person can be selected in 6 ways (any men or women can be selected), next one can be selected in 3 ways (i.e. opposite gender of the person selected in the first chair), next set can be taken by 2 remaining from an opposite gender from the 2nd chair, so 2 ways, 4th seat again can be filled by 2 remaining from opposite gender from the 3rd chair, last 2 chairs can be filled 1 way each. So no of ways:

6*3*2*2*1*1 = 72 ways. Is that correct ?[/quote]

Thx gayathri for the article, forgot abt circular prob, ofcourse, 72 needs to be divided by 6 ppl = 72/6 ==> 12 ways.

Another way of looking at..
The 1st person has 1 position only,not 6..On a circular table all the seats r the same..It doesn't make a lot of sense to get to 72 and then divide it by 6! Or so i think..
Good luck to all ..

In how many ways can 3 men and 3 women be seated around a [#permalink]
02 Jul 2015, 09:28

The answer GMAT wants is 12. You fix one guy to a seat and alternate man-woman. Now, the other two guys can be arranged in only two different ways with respect to that one guy you fixed in one seat. So, that's 2! ways. The women can be arranged in 3! ways with respect to the one guy. So, it's 2!*3!.

However

If each chair itself is unique, then shouldn't the answer be 72? The question assumes that the seats themselves don't matter, and that only the relative arrangement of people matters. So, if the seats actually mattered, say, seat A is a terrible chair that's about to fall apart, seat B is an Eames lounge chair, seat C is a massage chair, seat D is closest to the window, seat E is closest to the bathroom, and seat F is next to a different loud table, then such a scenario does exist, and the answer would need to be 72.

So, calculate 12 as you did above from one fixed guy in seat A, then move the guy you fixed to seat B, then C, then D, etc. That's 72 combinations, since each seat with respect to one guy has 12 arrangements and there are six seats total (12*6). Another way to do it is to think of each seat as M1-W2-M3-W4-M5-W6 (man seat#; woman seat#) around a table and multiply the combination, so after simplyfing it's 3*3*2*2*1*1 = 36. Those are only for the odd numbered seats for men, and even numbered seats for women, so the answer needs to be double that, giving us 72.

If 12 and 72 were both answer choices, I'd pick 12 but with serious protest. GMAT may ask for unique orders of people only, so the same order but in different chairs may be disqualified in GMAT's eyes. But, the GMAT doesn't specify that they want only unique orders of people - is it somehow implied in the question? The question itself is insufficient to draw a conclusion, esp. if 12 and 72 are both answer choices. Any thoughts?

Re: In how many ways can 3 men and 3 women be seated around a [#permalink]
09 Jul 2015, 01:36

Answer could be 4. first solve the question without constraint given .then it would be like this .. 3!(3-1)!=12. hence there are 12 ways .what can we infer from this is that the answer will be definately less than 12 if we add the constraint. so mens places are fixed here.as the arrangement is round so there will be (3-1)! ways =2 ways after this women position should be between two men.so here also there will be three places remaining .as the arrangement is round there will be (3-1)! ways=2 ways. therefore total of 4 ways..

Re: In how many ways can 3 men and 3 women be seated around a [#permalink]
09 Jul 2015, 06:32

GMATinsight wrote:

iPen wrote:

The answer GMAT wants is 12. You fix one guy to a seat and alternate man-woman. Now, the other two guys can be arranged in only two different ways with respect to that one guy you fixed in one seat. So, that's 2! ways. The women can be arranged in 3! ways with respect to the one guy. So, it's 2!*3!.

However

If each chair itself is unique, then shouldn't the answer be 72? The question assumes that the seats themselves don't matter, and that only the relative arrangement of people matters. So, if the seats actually mattered, say, seat A is a terrible chair that's about to fall apart, seat B is an Eames lounge chair, seat C is a massage chair, seat D is closest to the window, seat E is closest to the bathroom, and seat F is next to a different loud table, then such a scenario does exist, and the answer would need to be 72.

So, calculate 12 as you did above from one fixed guy in seat A, then move the guy you fixed to seat B, then C, then D, etc. That's 72 combinations, since each seat with respect to one guy has 12 arrangements and there are six seats total (12*6). Another way to do it is to think of each seat as M1-W2-M3-W4-M5-W6 (man seat#; woman seat#) around a table and multiply the combination, so after simplyfing it's 3*3*2*2*1*1 = 36. Those are only for the odd numbered seats for men, and even numbered seats for women, so the answer needs to be double that, giving us 72.

If 12 and 72 were both answer choices, I'd pick 12 but with serious protest. GMAT may ask for unique orders of people only, so the same order but in different chairs may be disqualified in GMAT's eyes. But, the GMAT doesn't specify that they want only unique orders of people - is it somehow implied in the question? The question itself is insufficient to draw a conclusion, esp. if 12 and 72 are both answer choices. Any thoughts?

You are absolutely correct about both your calculations however there here is a concrete point to know about GMAT

You don't consider Chairs different until GMAT clearly mentions it and No mention of the same would be considered that the arrangement takes into account only the positions at which people sit and not the chairs on which people sit

Re: In how many ways can 3 men and 3 women be seated around a [#permalink]
11 Jul 2015, 09:39

Deal with men and women separately:

Seating Layout: W M W M W M

Because they are in a circle, women can sit in (n-1)! ways as you can imagine them all getting up and moving one seat to the side and they would be in the same order.

Now that the women are in place the men need to be dealt with as if their positions were fixed because they are no longer sitting relative only to each other, but now are sitting relative to the women. The men can sit in n! ways.

Re: In how many ways can 3 men and 3 women be seated around a [#permalink]
27 Jul 2015, 20:19

I have one doubt if we are fixing one man's seat that is we are going to select that one man from the 3 man's available then it should be 3 why we are choosing 1?

Pls clear my doubt

gmatclubot

Re: In how many ways can 3 men and 3 women be seated around a
[#permalink]
27 Jul 2015, 20:19

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