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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!
Old 07-24-2007   #2
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Well, I will admit that I don't know the answer. I'm not very good with probability. However, I do know that the SECOND one is wrong. Why? Because you multiply by 0. And what happens when you multiply by 0? You get zero. And what can you NOT do with 0? Divide. Thus your second answer is not correct.
Old 07-24-2007   #3
rohan v
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no if you include the zero everything will become zero, so just go till 1 and that is perfectly fine
Old 07-24-2007   #4
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n= 7, r = 47C4 = 7!/(7-4)! 4!7C4 = (7x6x5x4x3x2x1) / (3x2x1) (4x3x2x1)7C4 = 5040 / 6x24 = 5040/144 = 35
Old 07-24-2007   #5
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(7!) over (7-4)!4!=7x6x5x4x3x2x1 over (3x2x1)x4x3x2x1

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