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 Post subject: Combinatronics Question #79Posted: Sat Apr 09, 2011 2:11 pm

Joined: Thu Mar 10, 2011 10:43 pm
Posts: 1
Hi,

After reviewing the explanation of this answer (choice E - 720), I am slightly confused by the formula used. I understand that this is a permutation problem with a restriction and the remaining problem is 6 seats for 4 people so 6PN. However, the explanation writes the equation as follows: 6!/(4-2)! where does this come from? I understand that 6!/(4-6)! gives 6!/(-2)! and negative factorials do not exists, but where does (4-2)! come from?

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 Post subject: Re: Combinatronics Question #79Posted: Wed Apr 13, 2011 5:51 pm

Joined: Thu Feb 12, 2009 6:32 pm
Posts: 497
If Jake drives, we have 6*5*4*3 possible arrangements ... 7 seats, one taken (the driver), so 6 choices for Lena * 5 choices for Fred, etc.

If Lena drives, we have 6*5*4*3 possible arrangements ... 7 seats, one taken (the driver), so 6 choices for Jake * 5 choices for Fred, etc.

So, the answer is 6*5*4*3 + 6*5*4*3

The formula is N!/(N - K)! ... N seats, K people to put in to those seats

6!/(6 - 4)! = 6*5*4*3

- Veritas Help

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