Complement rule

Written in symbols, $P(A^c) = 1 - P(A)$, where $A^c$ is read A complement and means A does not occur. It works because an event and its complement cover the whole sample space without overlapping, so their probabilities must add to 1. If the probability of rain tomorrow is 0.35, the probability of no rain is $1 - 0.35 = 0.65$. The rule is often used backwards as $P(A) = 1 - P(A^c)$ when the complement is the easier side to count.