Example: Rolling a "5" or "6"
Event A: {5, 6}
Number of ways it can happen: 2
Total number of outcomes: 6
P(A) = 2 = 1
6 3
The Complement of Event A is {1, 2, 3, 4}
Number of ways it can happen: 4
Total number of outcomes: 6
P(A') = 4 = 2
6 3
Let us add them:
P(A) + P(A') = 1 + 2 = 3 = 1
3 3 3
Yep, that makes 1
It makes sense, right? Event A plus all outcomes that are not Event A make up all possible outcomes.