Probability and the Addition Rule

Hi. I'm back with another problem. This one is:

If A, B, and C are mutually exclusive events with P(A) = .2, P(B) = .3, and P(C) = .4, determine P(A’ and B’ and C’).

I'm confused because according to the rule, P(A and B) = 0 for mutually exclusive events. Then I remembered DeMorgan's Law: (A and B)' = A' or B'.

My question is which law do I use and why? Thanks!

If A, B, C are pairwise mutually exclusive events then \(\displaystyle P(A \cup B \cup C) = P(A) + P(B) + P(C)\).
As you have notated \(\displaystyle (A \cup B \cup C)' = A' \cap B' \cap C'\).