J. Erwine is giving away some of his books.
It’s like one of those radio call -ins: Listener # 17 gets the free tickets. In this case if you are requester 0 modulo 10 for the set of positive numbers (sorry, I’m a math geek; that’s #10,#20 or #30) then you get a book.
I already tried so actually your odds are now increased (another math thing).