Complementary Events

An event’s complement is everything in the sample space that is not in the event. This NSW Year 7 maths subtopic (NESA outcome MA4‑PRO‑C‑01) shows that any event and its complement always have probabilities that add to 1.

You will use the complement rule, \(P(\text{not } A) = 1 - P(A)\), with fractions, decimals and percentages: a \(0.08\) chance of rain in Alice Springs means a \(0.92\) chance of no rain. You will also find a missing probability when several outcomes are known, using bags of marbles, playing cards and spinners. This subtopic is part of Year 7 Probability.