The complement of event A, written A′ or Aᶜ, is the event "A does not happen". If A is "roll even", A′ is "roll odd".
The complement rule says: P(A′) = 1 - P(A).
If P(even) = 1/2, then P(odd) = 1 - 1/2 = 1/2. If P(rain tomorrow) = 0.3, then P(no rain) = 0.7.
This rule saves time when it is easier to count what you do not want than what you do want.