Find the complement of a probability — the chance that an event does NOT happen.
Since something must either happen or not happen, the two probabilities always add up to exactly 1.
Complement: 1 − P(event)
Why is the complement useful?
Sometimes it is easier to find the chance of something NOT happening, and subtract that from 1, instead of working out the event directly. This is common in 'at least one' probability problems.