A village hall runs a small lending library which opens only on Saturdays. There are 80 members, each of whom may borrow up to four books at a time, which they may keep for three weeks. Members never overrun the three week time limit. After the annual stock-take, for which all books were returned, the library has 2000 books. On the next three Saturdays 120, 80, and 60 books are borrowed respectively.