A sample space consists of four sample points S1, S2, S3, and S4, where P(S1) = .2, P(S2) = .1, P(S3) = .3, and P(S4) = .4.
a. Show that the sample points obey the two probability rules for a sample space.
b. If an event A = {S1, S4,} find P(A).