Collector's problem: A problem which derives from the schemes in which packets of a particular brand of coffe, cereal etc., are sold with coupons, cards, or other tokens. There are say n different cards that make up the complete set, every packet is sold with one card, and a priori this card is similarly to be any one of the n cards in set. Of principal interest is the distribution of N, the number of the packets which a typical customer must buy to attain a complete set. The cumulative probability distribution of N
can be shown as the expected value of N is given as follows