Combine events using disjoint regions and overlap
Read union, intersection and complements, then add regions once rather than double-counting shared outcomes.
Exam scope and assessment notes
Probability & Statistics 1 (Paper 5, AS & A Level) · Mathematics 9709, 2026–2027, section 5.3. Define the sample space and state its probability assumptions before counting. Fair independent dice give equiprobable ordered pairs; their sums are not equiprobable. Random selection means each specified individual is equally likely under the stated sampling model. Without replacement, later branch probabilities change; with replacement and independent mixing, they remain the same. A′ is a complement, A ∩ B means both, A ∪ B means either or both, and P(A|B) means A given B with P(B) > 0. Mutually exclusive and independent are different properties. The explicit general addition formula is not required by this section; explain overlap through disjoint regions. Conditional probabilities may use P(A|B) = P(A ∩ B)/P(B). Numerical models describe the stipulated experiment, not automatically a real population. Written practice is self-checked; human subject-teacher review remains pending.
Before you begin
- Count finite sample spaces.
- Use complements and distinguish overlapping sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which region represents A ∩ B?
What you will learn
- Interpret A ∪ B, A ∩ B and A′.
- Combine probabilities by partitioning exclusive and overlapping events.
Or includes both unless stated otherwise
A ∪ B means A or B or both; A ∩ B means both. In a twenty-outcome equiprobable model, A-only has seven outcomes, both has five, B-only has four and neither has four. The union has 7 + 5 + 4 = 16 outcomes, so P(A ∪ B) = 4/5. The intersection probability is 5/20 = 1/4.
Adding the full A count 12 and full B count 9 gives 21 because their five shared outcomes were counted twice. Instead add the three disjoint union regions once. The explicit general addition formula is not required by this syllabus section, but understanding why overlap needs correction is essential.
Mutual exclusion removes the overlap
If two events cannot happen together, they are mutually exclusive and their union probability is the sum of their probabilities. On one fair die, ‘result 1’ and ‘result 6’ have union probability 1/6 + 1/6 = 1/3. This does not establish independence: mutual exclusion and independence answer different questions.
On one fair die let A be even and B be greater than 4. A = {2,4,6}, B = {5,6}. The union is {2,4,5,6}, with probability 2/3, and intersection {6} has probability 1/6. Since the overlap is possible, these events are not mutually exclusive. An ‘exactly one’ event would omit the intersection and has probability 3/6 = 1/2.
Pause and explain
Counts 7 A-only, 5 both, 4 B-only, 4 neither: what is P(A ∪ B)?
Worked example
Twenty equiprobable outcomes have counts A-only 7, both 5, B-only 4, neither 4. Find union and intersection probabilities.
Show the worked solution
- Check the disjoint counts sum to 20.
- Union uses A-only, both and B-only once: 16/20 = 4/5.
- Intersection uses the five shared outcomes: 5/20 = 1/4.
Answer P(A ∪ B) = 4/5; P(A ∩ B) = 1/4.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
In twenty equiprobable outcomes, region counts are A-only 7, both 5, B-only 4, neither 4. Find P(A ∪ B) and P(A ∩ B).
Give me a hint
Add disjoint union regions once, then use the shared region alone.
Compare my reasoning
- All four regions sum to 20.
- Union probability is (7 + 5 + 4)/20 = 4/5.
- Intersection probability is 5/20 = 1/4.
Union 4/5; intersection 1/4.
Look for these in your work
- Use the total experiment as denominator.
- Count the shared region once.
On one fair die, A is even and B is greater than 4. Find union and intersection probabilities and decide whether the events are mutually exclusive.
Give me a hint
List the sets and identify the shared result.
Compare my reasoning
- A = {2,4,6} and B = {5,6}; the shared result is 6.
- Union {2,4,5,6} gives 2/3 and intersection {6} gives 1/6.
- The events are not mutually exclusive because result 6 satisfies both.
Union 2/3; intersection 1/6; not mutually exclusive.
Look for these in your work
- Interpret or inclusively.
- Test exclusion by whether both can occur.
Mutually exclusive events A and B have probabilities 0.40 and 0.35. Find P(either event) and P(neither), stating why addition is valid.
Give me a hint
There is no shared probability to count twice.
Compare my reasoning
- Mutual exclusion means P(A ∩ B) = 0.
- The disjoint union probability is 0.40 + 0.35 = 0.75.
- Neither is the complement of the union: 1 − 0.75 = 0.25.
Either 0.75; neither 0.25.
Look for these in your work
- Use the explicit mutual-exclusion assumption.
- Take the complement of the whole union.
Common mistakes
- Inclusive or keeps the intersection.
- Do not add full overlapping event counts without a correction.
- Mutually exclusive does not mean independent.
What can you explain now?
How does exactly one of A and B differ from A ∪ B?
Compare with the explanation
Exactly one excludes the intersection, while the union includes it.
Use only A-only and B-only for an exactly-one condition.
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Make it stick
Draw or list disjoint regions; a union includes both, and an exactly-one condition excludes both.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.