Algebra - Discrete Mathematics

(2) Let S be the number of selections needed until every available colour has been selected by the little girl. For 0 g 72 g N 1, let X; be the random variable that after obtaining 3' different colours, the number of extra selections needed until further receiving a new colour. By studying the distributions of these Xi's as well as their relationship with 5', show that 1 1 1E[S]=N>