Hello, Can someone please tell me whether I can use negative binomial distribution for this question. "If there are 3 types of books in a bookstore and each book has an equal probability of being bought. What is the expected number of purchases to get all 3 books?" Using negative negative binomial, i get E(Y)=3/(1/3)=9. Is it just supposed to be this or is there something else to a question like this? Any help would be great. Thank you in advance.