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There are 9 books on a shelf, 7 hard cover and 2 soft cover. [#permalink]

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02 Oct 2003, 07:48

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There are 9 books on a shelf, 7 hard cover and 2 soft cover. How many different combinations exist in which you choose 4 books from the 9 and have at least one of them be a soft cover book?
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C(9,4) = 9!/(4! * 5!) = 126
So, that means...this will look like this:
9*8*7*6*5/4*3*2*5*4*3*2*1...right??
but calculate it.......its not 91
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why don't u try the permutation/combinations stuff on this website.
U can also consult any basic book on combinations/permutations or search on google for some stuff. I guess someone posted in share GMAT expe. section a link to some basic permutation/combo site. Check it out
Remember, u would want to be thorough in this area cause more and more questions based on them have begun to appear on GMAT
also 9! = 1*2*3...*8*9
it's pretty late here... time to sleep...
nowadays even in sleep i get these tormenting SC's...
thanks