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Old 07-24-2007   #1
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Default Math: Probability - Combinations?

My math final is tomorrow and I'm stressing about PROBABILITY. i fully understand permutation. but i'm having some trouble with combination.So the formula for combinations is n! over (n-r)!r!I know that. But maybe i just don't understand all of it.For example:One of my questions is 7C4 I did 7 x 6 x 5 x 4 over 4 x 3 x 2 x 1but then i remembered you're supposed to subtract N from R. So I subtracted 7-4 and got 3. THAT'S WHERE I GOT CONFUSED! Am I supposed to do it like this?: 7 x 6 x 5 x 4 over 3 x 2 x 1 x 0?I'm not sure if I'm supposed to include zeros.HELP ME PLEASE!