When ordering a personal computer, a customer can add the following features to the basic configuration: (1) additional memory, (2) flat panel display, (3) professional software, and (4) wireless modem. A random computer order has feature i with probability pi = 2-i independent of other features. In an hour in which three computers are ordered, let Ni equal the number of computers with feature i. (a) Find the joint PMF
(b) What is the probability of selling a computer with no additional features?
(c) What is the probability of selling a computer with at least three additional features?