Permutation and Combination 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.
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.
I have 100 books for 100 users, I want to distribute 1 book to 1 user for 1 week. How many weeks I can distribute all the 100 books to all the 100 users without repetition of the book to the users?

What is the formula to calculate this?

Thanking you for the help.