A mail-order computer business has six telephone lines. Let X denote the number of lines in use at a specified time. Suppose the pmf of X is as given in the accompanying table. x 0 1 2 3 4 5 6 p(x)...

A mail-order computer business has six telephone lines. Let X denote the number of lines in use at a specified time. Suppose the pmf of X is as given in the accompanying table.

a. Find the probability that at least four lines are in use.

b. Let F be the cumulative distribution function.

Find  F(3) , F(-1), F(2.5),  F(8)