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Smita04
Four female friends & four male friends will be pictured in a advertising photo. If the photographer wants to line them up in one row, with men & women alternating. How many possible arrangements may she chose?

A. 40320
B. 1680
C. 1152
D. 576
E. 70

To meet the condition in the stem men and women should line up either: M-W-M-W-M-W-M-W or W-M-W-M-W-M-W-M. Now, for each of these two cases men can be arranged in 4! ways and women can be arranged also in 4! ways, hence total # of arrangements is 2*4!*4!=1,152.

Answer: C.

Hope it's clear.




Why can't it be 8!/4!*4! = 70?

Can you please explain?
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Smita04
Four female friends & four male friends will be pictured in a advertising photo. If the photographer wants to line them up in one row, with men & women alternating. How many possible arrangements may she chose?

A. 40320
B. 1680
C. 1152
D. 576
E. 70

To meet the condition in the stem men and women should line up either: M-W-M-W-M-W-M-W or W-M-W-M-W-M-W-M. Now, for each of these two cases men can be arranged in 4! ways and women can be arranged also in 4! ways, hence total # of arrangements is 2*4!*4!=1,152.

Answer: C.

Hope it's clear.




Why can't it be 8!/4!*4! = 70?

Can you please explain?

8!/(4!*4!) is the number ways to arrange 8 items out of which 4 are of type A and 4 are of type B. For example, the number of ways to arrange 33334444 is 8!/(4!4!).
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There is some problem with the answer i guess. Correct me if I am wrong.
SO we have 4 boys and 4 girls such that no two boys or no two girls sit togethe.
So we can do this question by gaping method: _B_B_B_B_
We have 5 places for four girls. So we can choose 4 places using 5C4.
Next we can arrange boys and girls in 4!*4! ways.

So the correct answer should be: 5C4*4!*4! = 2880.
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jim441
There is some problem with the answer i guess. Correct me if I am wrong.
SO we have 4 boys and 4 girls such that no two boys or no two girls sit togethe.
So we can do this question by gaping method: _B_B_B_B_
We have 5 places for four girls. So we can choose 4 places using 5C4.
Next we can arrange boys and girls in 4!*4! ways.

So the correct answer should be: 5C4*4!*4! = 2880.


5 places for 4 girls allows 1 place to be unfilled. That one place could be any of the spots between the boys, which would mean two boys sitting next to each other, violating the question stem.

Posted from my mobile device
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Regor60
jim441
There is some problem with the answer i guess. Correct me if I am wrong.
SO we have 4 boys and 4 girls such that no two boys or no two girls sit togethe.
So we can do this question by gaping method: _B_B_B_B_
We have 5 places for four girls. So we can choose 4 places using 5C4.
Next we can arrange boys and girls in 4!*4! ways.

So the correct answer should be: 5C4*4!*4! = 2880.


5 places for 4 girls allows 1 place to be unfilled. That one place could be any of the spots between the boys, which would mean two boys sitting next to each other, violating the question stem.

Posted from my mobile device

Thanks a lot, this one my really messing up with me.
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Smita04
Four female friends & four male friends will be pictured in a advertising photo. If the photographer wants to line them up in one row, with men & women alternating. How many possible arrangements may she chose?

A. 40320
B. 1680
C. 1152
D. 576
E. 70

The word "arrangements" indicates permutations. There are a couple of ways to do this problem.

Method A: Permutations

There are 4 women and 4 men, and the pattern they're sitting in can be W M W ... or M W M.... So arrangement would be \(4!*4!*2=1152\).

Method B: Filling seats

You could also do this using counting principle, where you multiply together the options for each seat. This would result in:

\(4*4*3*3*2*2*1*1=576\)

But because there are two patterns (W M W or M W M), need to multiply by 2, which gives 1152.
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