Mathematics·Probability·conceptual
Probabilities Sum to One
Understand that when all possible outcomes of a trial are listed, their probabilities must add up to 1; use this to find the probability of an event NOT happening: P(not A) = 1 − P(A); apply this shortcut to avoid counting all unfavourable outcomes directly
Suggested ages 10–11
Evidence of understanding
- List all outcomes of spinning a 4-colour spinner and verify their probabilities add up to 1
- Calculate P(not rolling a 3) as 1 − 1/6 = 5/6 using the complement rule
- Spot an error in a probability table where the values don't sum to 1 and explain what's wrong
Assessment prompt
If the probability of it raining tomorrow is 0.3, can Probabilities Sum to One work out the probability of it NOT raining — and explain why all probabilities in a situation must add up to 1?
Standards alignment
No external standards are linked to this topic.