Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|cite|improve this question
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.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.