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Probabilities question

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allenkt Senior | Next Rank: 100 Posts Default Avatar
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Probabilities question

Post Sun Apr 29, 2007 7:08 pm
Admittedly, probability is a weak area for me because I don't remember ever taking anything like it in high school or college (never took statistics). I have a question about a formula. I recently learned the formula to be as follows:

If you have n items, taken r at a time, the total number of possible combinations can be found as such:

C(n,r) = n! / r! (n-r)!

However in most of the problems I've seen nobody is using that exact formula. Instead they are using something else. I can't remember it exactly and I can't seem to find an example but it's something like

n!/r! * n!

What is difference in the two ways of finding the answer? Hopefully someone knows what I'm talking about because I'll admit I'm even starting to confuse myself...lol.

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Stacey Koprince GMAT Instructor
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Post Mon Apr 30, 2007 1:28 pm
The formula you list below is for combination / permutation problems, which sometimes overlap with probability, but they aren't the same thing. I don't recognize your second formula - but the first is the classic combination formula (though there are a lot of others for both comb. and perm.).

There are also a lot of methods to choose from in this area - and they're too involved for me to type out right here, unfortunately. If you are taking a course, ask your teacher about comb/perm in particular and how best to tackle these. If you are studying on your own, find combinations / permutations in whatever books you are using and use that method. (And if you have more than one book, pick the method that is most comfortable for you.)

These don't show up a ton on the test, so don't worry about them too much.

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