Combination with replacement?

[americanpierg]

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Any know the formula? Too late (early?) and i seem to have lost my googling skills. Plus if ur smart, can you explain the formula. TY

 

[solstice]

http://forums.studentdoctor.net/showthread.php?t=654696&highlight=permutation

 

[americanpierg]

http://forums.studentdoctor.net/showthread.php?t=654696&highlight=permutation

Actially that thread doesn't help, it just goes over the elementary stuff.

I found the answer anyways.

Dr. Math was very informative in his answer, but anways the equation for combination with repetetion is

(n + r - 1)! / (r!(n-1)!) - Choosing 5 plates of Arby's 5 for $50 from 8 possible dishes

another interesting find is that the sum of all possible combinations is
2^n - total number of possible topping combinations on a pizza with 8 different topping choices


pretty cool stuff that i could actually even use in real life unlike trying to find out why a strong tertiary alcohol base leads to a non-zaitzev elimination reaction