Estimated wait time for a library book A library has 20 copies of a book that circulates for 3 weeks. All copies are out and there 40 people on reserve ahead of me. How do I estimate the wait time?

If we assume that all books were given away just as you joined the queue (highly unlikely), you'd have to await further two full cycles ($40/20$) until you are first and then wait another cycle for the books to be returned, making a total of $3$ cycles, i.e. $9$ weeks.  
This is extremely unrealistic, though. You should probably assume a certain distribution with expectation $3$ (in weeks) i.i.d. per book and see what the expected time until $60$ "returns" happen, noting that after each return, the distribution "recovers" per book.
