A mail-order computer software business has six telephone lines. Let x denote the number of lines in use at a specified time. The probability distribution of x is as follows:

X 0 1 2 3 4 5 6

P(x) 0.13 0.17 0.20 0.25 0.15 0.08 0.02

Calculate the probability of:

a. At most three lines are in use.

b. Fewer than three lines are in use.

c. At least three lines are in use.