13. 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.

Calculate the probability of each of the following events.

a. {at most three lines are in use}

b. {fewer than three lines are in use}

c. {at least three lines are in use}

d. {between two and five lines, inclusive, are in use}

e. {between two and four lines, inclusive, are not in use}

f. {at least four lines are not in use}