Probability: single and combined events

Key learnings

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  All the probabilities add up to 1

  For one trial, the probabilities of all possible outcomes sum to $1$. Use this to find a missing probability: $P(\text{missing}) = 1 - (\text{the rest})$.

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  Expected frequency = probability × number of trials

  To estimate how many times an outcome happens in $n$ trials, multiply: $\text{expected} = P(\text{event})\times n$.

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  Mutually exclusive "or": add the probabilities

  If two events cannot both happen, $P(A\text{ or }B)=P(A)+P(B)$.