There are c different types of coupon and each time you obtain a coupon it is equally likely to be any of the c types. Let Yi be the additional number of coupons collected, after obtaining i distinct types, before a new type is collected.
(a) Show that Yi has the geometric distribution with parameter (c-i)/c
(b) Deduce from this the mean number of coupons you will need to collect before you have a complete set. (c) Find the expected number of different types of coupon in the first n coupons received.