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Mathematics · Grades 7–10

Complementary probability

An event and its complement cover all outcomes without overlap.

Make a worksheet

The key idea

An event and its complement cover all outcomes without overlap.

P(not A) = 1 − P(A)

Worked examples

P(A) = 8/20. Find P(not A).

1 − 8/20 = 3/5

3/5

P(A) = 3/18. Find P(not A).

1 − 3/18 = 5/6

5/6

Try it yourself

  1. P(A) = 3/11. Find P(not A).

  2. P(A) = 2/6. Find P(not A).

Check your understanding

Answers and marking notes
  1. 8/11

    1 − 3/11 = 8/11

  2. 2/3

    1 − 2/6 = 2/3

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.