I need help on this statistic problem tnx?

Question

2.An insurance company offers its policyholders a number of different premium payment options. For a randomly selected policyholder, let X be the number of months between sucessive payments. The cumulative distribution function of X is

F(x)= {0, if x<1,

0.4, if 1<=x<3,

0.6, if3<=x<5,

0.8, if5<=x<7,

1.0, ifx>=7.

a. What is the probability mass function of X?

b. Compute P(4<X<=7)

Answer 1

f(x) =

0.4 .. . . x = 1

0.2 .. . . x = 3, 5, 7

P(4 &lt; X ≤ 7) = P(X = 5) + P(X = 7) = 0.4