Basic probability
A standard method from Probability: the steps and worked examples.
Steps
P(E) = number of favourable outcomes ÷ total number of equally likely outcomes.
- 0 ≤ P(E) ≤ 1.
- P(not E) = 1 − P(E).
Worked examples
Example 1: NF: a game costs $10. You win $50 with probability 1/8 and $20 with probability 1/4; otherwise you win nothing.
expected winnings = 50 × 1/8 + 20 × 1/4 = 6.25 + 5 = $11.25
expected net gain = 11.25 − 10 = $1.25 per gameFor the whole topic, read Probability: key points, common mistakes and how the DSE tests it.