Probability measures how likely something is, from 0 (impossible) to 1 (certain). When all outcomes are equally likely, P(event) = number of ways it can happen ÷ total number of outcomes.
A bag has 2 green, 6 red and 4 blue counters. There are 12 counters, so P(red) = 6/12 = 1/2. The probabilities of all the outcomes add up to 1, so P(not red) = 1 − 1/2 = 1/2.
Worked example
Cards numbered 1 to 10 are shuffled and one is taken. What is the probability it is a multiple of 3?
- Multiples of 3: 3, 6, 9, which is 3 outcomes.
- There are 10 cards altogether.
Answer:3/10
Key words
- outcome
- one possible result
- event
- the result you are interested in
- probability
- a number from 0 to 1 showing how likely
Practise this
New questions every time.
Common Core-referenced: 7.SP.C.5, 7.SP.C.7