Which law?
A standard method from More about probability: the steps.
Steps
| Situation | Law |
|---|---|
| "or" | P(A ∪ B) = P(A) + P(B) − P(A ∩ B). If mutually exclusive, P(A ∩ B) = 0. |
| "and", independent | P(A ∩ B) = P(A) × P(B) |
| "and", not independent (second depends on first) | P(A ∩ B) = P(A) × P(B | A) |
| "at least one" | 1 − P(none) |
| choosing several items at once | P = (ways for the event) ÷ (all ways), both with ⁿCᵣ |
- Underline the key words: "or", "and", "given", "at least one".
- For "and", ask whether the first result changes the chances of the second. If it does, use P(B | A).
- Pick the law from the table and write it down before putting in the numbers.
For the whole topic, read More about probability: key points, common mistakes and how the DSE tests it.