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Section 13.2 · Statistics and probability

Calculating probabilities

Calculate the probability of an event with equally likely outcomes, and use the fact that an event and its complement total 1.

6 teaching slides 50 practice questions Worked answers for every question

1 · Teaching slidesuse ← → keys

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2 · Key words

equally likelyEvery outcome has the same chance.
favourable outcomeAn outcome that counts as the event happening.
complementThe event not happening.
mutually exclusiveTwo events that cannot both happen.
trialOne go: one roll, one spin, one draw.
randomChosen so every outcome has an equal chance.

3 · Summary of the sectionrevise from this

  • When outcomes are equally likely: P = favourable outcomes ÷ total outcomes.
  • Count the total carefully — it is the denominator of every answer in the question.
  • The probabilities of all possible outcomes add to 1, because something must happen.
  • So P(not A) = 1 − P(A). This is often much quicker than counting.
  • Two events are mutually exclusive if they cannot both happen.
  • For mutually exclusive events, P(A or B) = P(A) + P(B) — no outcome gets counted twice.
  • Give the answer as a fraction in its simplest form unless the question says otherwise.
  • The formula only applies when the outcomes really are equally likely: a drawing pin is not a fair coin.
PDFPrintable worksheet All 50 questions, space to work PDFWorked answers Full solutions, not just the answer

Video lessons1 videos

Probability

British corbettmaths

Favourable over total, with the counting done carefully.

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4 · Practice50 questions, easiest first

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13.1 The probability scale
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13.3 Experiments and expected results