# Probabilities Sum to One

Understand that when all possible outcomes of a trial are listed, their probabilities must add up to 1; use this to find the probability of an event NOT happening: P(not A) = 1 − P(A); apply this shortcut to avoid counting all unfavourable outcomes directly

Builds on: The 0-to-1 Probability Scale (hard, https://ai.page/c/the-0-to-1-probability-scale/en.md), Calculating Simple Probability (soft, https://ai.page/c/calculating-simple-probability/en.md)

Concept id: probabilities-sum-to-one. Subject: maths. Licence: CC BY-SA 4.0.

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