2.An insurance company affers 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)
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