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I'm sure you can get information on factorials from several of the books that people in the forums and/or BB has mentioned OR you can probably get it from the permutation, counting, and probability tutorial document(s)that is offered elsewhere on this site.
But, honestly, factorials in it of themselves aren't that complicated. Just remember that -- for any whole number n:
Remember that you may be able to use smaller factorials to cancel out parts of larger factorials when dividing factorials.
There are some more complicated rules regarding factorials in more advanced mathematics, but those concepts will NEVER come up on the GMAT (those concepts would only be fair to test on math majors and not with the US jr. high and high school level math tested on the GMAT).