What is important to know about Basic probability?

P(event) = number of favourable outcomes ÷ total number of equally likely outcomes.

Probabilities range from 0 to 1.

P(event does not happen) = 1 − P(event happens). This is the complementary rule.

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Statistics & Probability · Foundation & Higher

Basic probability

Probability measures how likely an event is to occur, on a scale from 0 (impossible) to 1 (certain).

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1P(event) = number of favourable outcomes ÷ total number of equally likely outcomes.
2Probabilities range from 0 to 1.
3P(event does not happen) = 1 − P(event happens). This is the complementary rule.
4If events are mutually exclusive: P(A or B) = P(A) + P(B).
5If events are independent: P(A and B) = P(A) × P(B).

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Basic probability

P(A) = favourable outcomes ÷ total outcomes

Complement

P(A') = 1 − P(A)

Independent events

P(A and B) = P(A) × P(B)

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Example 1

A bag has 3 red, 5 blue and 2 green balls. Find P(red) and P(not green).

Working

AnswerP(red) = 3/10, P(not green) = 4/5

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✗Forgetting to count the total number of outcomes.

✗Adding probabilities when you should multiply (for "and" use ×, for mutually exclusive "or" use +).

✗Not simplifying fractions.

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✓Always check probabilities add up to 1 for a complete set of outcomes.

✓Use fractions or decimals — never write "3 in 10" as a final answer without a proper fraction/decimal.

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