I'm currently learning permutation and combination in my discrete math class and I'm kind of lost. So I know this is the general set of formulas:
r-permutations no repetition n!/(n-r)! r-combinations no repetition n!/r!(n-r)! r-permutations repetition n^r r-combinations repetition (n+r-1)!/r!(n-1)!
But when do I know a problem allows repetition or not? This may sound like a really stupid question to people who already know this pretty well but I'm new to this and don't really understand.