A mail-order computer business has six telephone lines, Let X denote the number of lines in use at a specified time. Suppose the pit of X is as given in the accompanying table [6] 0 1 2 3 5 6 P(x) 01 0.15 0.2 0.25 0.2 0.06 0.04 Calculate the probability of each of the following events. a. {at most three lines are in use} b. {at least three lines are in use} c. (between two and five lines, inclusive, are in use} d. {at least four lines are not in use}