When two things happen, such as flipping a coin and rolling a die, list every possible pair of outcomes. A table called a sample space diagram helps you not miss any.
A coin and a die give 2 × 6 = 12 equally likely outcomes. Exactly one of them is heads and a 6, so P(heads and 6) = 1/12. For two dice, there are 36 outcomes; P(total 7) = 6/36 = 1/6, because six pairs add to 7.
Worked example
Two fair spinners numbered 1 to 3 are spun. What is the probability the total is 4?
- There are 3 × 3 = 9 outcomes.
- Totals of 4: (1, 3), (2, 2) and (3, 1).
Answer:3/9 = 1/3
Key words
- sample space
- a list of all possible outcomes
- equally likely
- all outcomes have the same chance
- combined event
- two or more things happening together
Practise this
New questions every time.
Common Core-referenced: 7.SP.C.8