In how many ways can 6 people, A, B, C, D, E, F be seated

Total # of arrangements of 6 people in a row is 6!;

Now, let's find # of arrangements where C and D as well as A and B ARE seated next to each other. Consider them as a single unit: {AB}, {CD}, E, F. Now, these 4 units can be arranged in 4!, but we should multiply this number by 2*2 as A and B, as well as C and D within their unit can be arranged in two ways: {AB} or {BA}, and {CD} or {DC}. Hence total for this case is 4!*2*2;

So, {# of arrangements}={total}-{restriction}=6!-4!*2*2=624.

Answer: D.
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This is very similar to a question I discussed in a post a week back. Let me give you the link of the post which explains the concept in detail. Perhaps you can check it out and see if you can figure out the answer of this one on your own! I have discussed two methods in it.

https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2012/01 ... e-couples/
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Number of total arrangements = 6!
Restriction 1= AB & CD not next to each other --> let say AB and CD are considered as one unit, respectively
Restriction 2= AB is not the same as BA + CD is not the same as DC --> the number will increase by 2*2

Total number of arrangements - Number out of restrictions = Result

6! - (4!*2*2) = 720 - (24*2*2) = 624

Answer D

6 people in the row is 6!=720

glue method is best , so consider A&B and C&D sit together

now we have 4!=24, but 2 people in one pair and 2 people in another pair can sit in 4 positions, so 24*4=96

desirable condition is 720-96=624

D

@Veritasprep

What is the difference between this question and the one mentioned in the link provided? They seem to be the same to me so why the difference in the approach and consequently, different answers?

Hi Bunuel,
In my opinion, this question translates into "neither A and B, nor C and D should sit together". I think that the correct answer is not mentioned in the options.
There are four cases possible

1. AB sit together and CD sit together
2. AB sit together but CD don't sit together
3. CD sit together but AB don't sit together
4. neither AB nor CD sit together.

We have to subtract 1,2, and 3 from total possible cases.

I have found the answer to be 336 in this manner.

Agreed ! The answer choices are incorrect. It should be 336.

The language might be misleading here but still I translated "as well as" as "and" not as "or".
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What if AB are together, but CD are separate. Similarly, if CD are together and AB are separate.
These two cases shouls aldo be taken into account.

Hi Bunuel,
In my opinion, this question translates into "neither A and B, nor C and D should sit together". I think that the correct answer is not mentioned in the options.
There are four cases possible

1. AB sit together and CD sit together
2. AB sit together but CD don't sit together
3. CD sit together but AB don't sit together
4. neither AB nor CD sit together.

We have to subtract 1,2, and 3 from total possible cases.

I have found the answer to be 336 in this manner.[/quote]

The language might be misleading here but still I translated "as well as" as "and" not as "or".[/quote]

That's not correct. I have worked on similar problems and I agree what ShashankDave has to say. I am 100% sure about it. I ave spoken to various maths experts, and they've told me the same thing!

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