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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.

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up vote 4 down vote accepted

Figuring out how to interpret a real world situation can be quite hard.I think that best approach is practice.On this page you can find some examples how to apply formulas on real world situations.

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