An insurance company offers its policyholders a number of different premium payment options. For a randomly selected policyholder, let X = the number of months between successive payments. The cdf of X is as follows:
F(x) = {
0 x < 1
.30 1 <= x < 3
.40 3 <= x < 4
.45 4 <= x < 6
.60 6 <= x < 12
1 12 <= x
a. What is the pmf of X?
b. Using just the cdf, compute P(3 <= X <= 6) and P(4 <= X).