I need help with MC I-B problems 8-20 and 8-28.
For both 8.20 and 8.28 you'll need a Venn diagram. A Venn diagram diagram with two sets splits in four mutually exclusive regions, as shown below
You can then write the probability of a set as a sum of the probabilities of these four regions. For example, $P(B) = P(A^C\cap B) + P(A\cap B)$, or $P(A^C) = P(A^C\cap B) + P(A^C \cap B^C)$.