Basically what I want to know is how you derive $$\frac{n!}{r!(n-r)!}$$ from the number of combinations of $r$ objects in n places. Essentially I couldn't work out why it was and just need an explanation. Any help appreciated. Thank you.


marked as duplicate by GNUSupporter 8964民主女神 地下教會, N. F. Taussig combinatorics Jan 14 at 22:54

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ there are n! total ways to permute the n objects. But you can permute the r indistinguishable objects in r! ways. Likewise with the (n-r)! spaces that are not occupied by anything. $\endgroup$ – Justin Stevenson Jan 14 at 22:43

Browse other questions tagged or ask your own question.