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A Level · Cambridge (CIE) · 9709

Counting and combined probability

Decide whether order matters and whether events are independent.

Probability & Statistics 1 · Counting and combined probability revision · 5.2 / 5.3
Exam scope and assessment notes

Probability & Statistics 1 (Paper 5) · Mathematics 9709, 2026–2027. This revision overview focuses on selections, assigned roles and event classification; it does not cover every arrangement restriction or all of sections 5.2–5.3. Written solutions use self-check criteria, not an official mark scheme. Specialist accuracy review has not been completed. The diagram uses three names as a separate illustration; use the numbers and random models specified in each task.

Before you begin

  • Use factorials and simplify fractions.
  • Interpret intersection as both events and recognise equally likely outcomes.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

Five candidates are available. Compare choosing two equal-status representatives with appointing a lead and a deputy, with nobody holding both roles. How many outcomes are there?

What you will learn

  • Use combinations for unordered selections.
  • Distinguish independent from mutually exclusive events.
Two names from A, B, C give six ordered pairs AB, BA, AC, CA, BC, CB but three unordered sets AB, AC, BC. Each unordered set has two descriptions in the ordered list. Thus 3P2 = 6 and 3C2 = 6/2! = 3.Does swapping names change it?Ordered roles: 6 pairsABBAACCABCCBUnordered members: 3 setsABACBCAB and BA name the same set6/2! = 3
Two names from A, B, C give six ordered pairs AB, BA, AC, CA, BC, CB but three unordered sets AB, AC, BC. Each unordered set has two descriptions in the ordered list. Thus 3P2 = 6 and 3C2 = 6/2! = 3.

Count outcomes consistently

A committee is a selection, so rearranging its members does not create a new committee. Use combinations to count selections. In contrast, choosing a first, second and third place distinguishes order and needs a different count.

A probability can be calculated as favourable outcomes divided by all outcomes only when the counted outcomes are equally likely and use a consistent counting method.

Independence is a condition

Independent events satisfy P(A∩B)=P(A)P(B). Mutually exclusive events cannot happen together, so their intersection has probability zero. Events with positive probabilities cannot be both independent and mutually exclusive.

For any two events, P(A∪B)=P(A)+P(B)−P(A∩B). Subtract the intersection so outcomes in both events are not counted twice.

C(n,r)=n!/[r!(n−r)!]

Sources for this lesson

Pause and explain

Events A and B have P(A) = 2/5, P(B) = 1/4 and P(A ∩ B) = 1/20. How are they related?

Put the idea to work

Worked example

A team of 2 is selected from 7 students. How many teams are possible?

Show the worked solution
  1. Order does not matter: choosing Ana then Ben is the same team as Ben then Ana.
  2. There are 7×6 ordered selections.
  3. Divide by 2! to remove duplicate orderings.

Answer 21 teams

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.

1 · Guided

Eight candidates form a four-person panel with one chair, one secretary and two equal-status ordinary members. Nobody holds two roles. Count the possible panels with roles, and check your answer by choosing the chair and secretary first.

Give me a hint

A panel membership set and an assignment of roles are different outcomes. Only the two ordinary members are interchangeable.

Compare my reasoning
  1. First choose four people in 8C4 = 70 ways. Choose the chair in 4 ways and the secretary from the remaining three in 3 ways. The other two need no ordering.
  2. The total is 70×4×3 = 840. Dividing all four roles by 4! would erase distinct chair/secretary assignments; multiplying the ordinary pair by 2 would count it twice.
  3. Alternatively choose the chair in 8 ways, the secretary in 7 ways and the two ordinary members from the remaining six in 6C2 = 15 ways. This gives 8×7×15 = 840.

840 panels with roles.

Look for these in your work

  • Separate membership selection from role assignment.
  • Account for exactly which positions are interchangeable.
  • Obtain the same total by two valid counting orders.
2 · Independent

Choose an unordered set of three tickets uniformly from eight distinct tickets numbered 1–8. Let A mean ticket 1 is included and B mean ticket 2 is included. Find P(A), P(B) and P(A ∩ B). Decide whether A and B are independent or mutually exclusive, and check using P(B | A).

Give me a hint

Count all three-ticket sets once. For the intersection, both named tickets are already fixed.

Compare my reasoning
  1. There are 8C3 = 56 equally likely sets. Sets containing ticket 1 number 7C2 = 21, so P(A) = 21/56 = 3/8; symmetry gives P(B) = 3/8.
  2. Sets containing both named tickets number 6C1 = 6. Thus P(A ∩ B) = 6/56 = 3/28. This is positive, so the events are not mutually exclusive. It differs from (3/8)² = 9/64, so they are not independent.
  3. Given A, the other two tickets are chosen uniformly from seven, so P(B | A) = 2/7. Equivalently 6/21 = 2/7. This is below 3/8: fixing one place for ticket 1 reduces the chance that ticket 2 is also included.

P(A) = P(B) = 3/8; P(A ∩ B) = 3/28; P(B | A) = 2/7. Neither independent nor mutually exclusive.

Look for these in your work

  • Use a consistent unordered sample space and state its size.
  • Compare intersection with both zero and the product of marginals.
  • Restrict the denominator correctly when conditioning on A.
3 · Transfer

A token is chosen uniformly from 12 distinct tokens numbered 1–12. A means its number is divisible by 3; B means divisible by 4; C means the number is 1 or 2. A learner claims that overlapping events must be dependent, whereas mutually exclusive events must be independent. Evaluate both claims by comparing A with B, then A with C.

Give me a hint

List event members before calculating. An example can refute a claim about all events.

Compare my reasoning
  1. A = {3, 6, 9, 12} and B = {4, 8, 12}. Their probabilities are 1/3 and 1/4, and their intersection is {12} with probability 1/12. This equals (1/3)(1/4), so A and B are independent despite overlapping.
  2. C = {1, 2} has probability 1/6. A and C have no common outcome, so they are mutually exclusive. But 0 differs from (1/3)(1/6) = 1/18, so they are not independent.
  3. Both claims fail. Overlap asks whether both events can occur; independence asks whether the joint probability equals the product. Two mutually exclusive events with positive probabilities cannot be independent.

A and B: independent, not mutually exclusive. A and C: mutually exclusive, not independent.

Look for these in your work

  • Enumerate event members and intersections correctly.
  • Test independence numerically rather than inferring it from overlap.
  • Explain why positive marginal probabilities matter in the mutual-exclusion argument.

Common mistakes

  • Do not multiply event probabilities unless independence or the appropriate conditional probability justifies it.
  • A committee is not an ordered arrangement.
Recall without your notes

What can you explain now?

Independent events have probabilities 0.3 and 0.5. Find the probability that both occur.

Compare with the explanation

0.15

Independence permits multiplication: 0.3×0.5=0.15.

After trying it yourself, choose your next review. This is your self-assessment.

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Make it stick

Explain when swapping selected people creates a new outcome. Then distinguish testing an intersection against zero from testing it against the product of the marginal probabilities. State how a conditional sample space changes the denominator.

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.

Keep learning Mathematics

Course scope and assessment

312 sequenced lessons provide 936 guided, independent and application problems covering the 160 objectives in sections 1.1–1.8, 2.1–2.6, 3.1–3.9, 4.1–4.5, 5.1–5.5 and 6.1–6.5. The map lists all 38 sections across six alternative components; all eight Pure Mathematics 1 sections and all six Pure Mathematics 2 sections, from Paper 2 Algebra through Numerical solution of equations, currently have full teaching sequences. Paper 3 Algebra, section 3.1, links the six shared foundation lessons plus six lessons on partial fractions and rational binomial expansions; Paper 3 logarithms, trigonometry and numerical equations use the shared lessons, with the Paper 1 graph foundation linked into trigonometry. Paper 3 differentiation combines the shared methods with its inverse-tangent derivative lesson. Paper 3 Integration links four shared standard and identity lessons plus seven advanced lessons, covering all six section 3.5 requirements. Eight Vectors lessons cover all six section 3.7 requirements, with 2D and 3D line and solid applications. Nine Differential equations lessons cover all four section 3.8 requirements, including Paper 3 integration techniques, initial conditions, equilibrium solutions and conditional model interpretations. Eleven Complex numbers lessons cover all eight section 3.9 requirements, with seven Argand diagrams and explicit argument and boundary checks. All nine Pure Mathematics 3 sections now have teaching linked to every mapped requirement. Nine Forces and equilibrium lessons cover all seven section 4.1 requirements, with six force diagrams, signed incline-friction intervals, reaction-dependent pull and push limits and separate-body third-law checks. Nine Kinematics lessons cover the four official section 4.2 requirements, with the graph requirement mapped to three relationships. Six motion diagrams support directed quantities, graph slopes and signed areas, Paper 1 time-calculus methods, reversal distances, constant-acceleration stages and shared-clock particle meetings. Six Momentum lessons cover both section 4.3 requirements, including signed system totals, conservation assumptions, outgoing velocities, coalescence, unknown masses and successive impacts. Four diagrams show system momentum through the stipulated events, and supplied kinetic-energy checks distinguish conservation of momentum from changes in kinetic energy. Twelve Newton’s laws lessons cover all four section 4.4 requirements, with vertical support and release stages, rough horizontal and incline motion, inferred parameters, hanging and surface pulley systems, rigid tow-bar thrust and slack-string transitions. Seven force diagrams support the separate-particle balances. Nine Energy, work and power lessons cover all five section 4.5 requirements, with angle-dependent work, kinetic and reference potential energy, signed net-work balances, smooth curved slides, rough and driven slopes, ideal connected-system energy and instantaneous power on hills. Five diagrams distinguish the energy inputs, losses and valid positive-speed force model. All five Mechanics sections now have teaching and staged practice linked to their mapped requirements. Thirteen Representation of data lessons cover all five section 5.1 requirements, including choice and limitations of displays, ordered and back-to-back stems, raw summaries and box plots, unequal-width density histograms, cumulative-frequency interpolation, descriptive moment calculations, grouped estimates, coding, combining two datasets and correcting totals. Six accessible diagrams retain repeated observations and distinguish numerical spread, frequency area and estimated readings. Ten Permutations and combinations lessons cover both official section 5.2 requirements, with selection and ordered roles, repeated objects, adjacency blocks, non-adjacency gaps, leading-zero cases, labelled multi-row seating and mixed restrictions. Six accessible diagrams show outcome trees, ordered versus unordered choices, repeated copies, blocks, gaps and distinct row seats. Eleven Probability lessons cover all four section 5.3 requirements, with equiprobable elementary outcomes, selection-based probabilities, complements and disjoint regions, independence tests, restricted sample spaces, two-way tables, replacement and removal trees, reversed conditions and unknown parameters. Seven accessible diagrams show dice outcomes, random subsets, event regions, a conditioned grid, a count table and two probability trees. Twelve Discrete random variables lessons cover all three official section 5.4 requirements, with complete probability tables, moment and parameter checks, binomial assumptions, exact probabilities and moments, geometric first-success probabilities, tails, expectation and integer reliability thresholds. Six accessible diagrams distinguish outcome grouping, weighted moments, fixed trials, discrete probability bars, waiting conventions and integer tail deadlines. Twelve Normal distribution lessons cover all three official section 5.5 requirements, with continuous-area interpretation, MF19 cumulative tables, full standardisation, tail and interval regions, conditional probabilities, percentiles, one- and two-parameter inference, binomial suitability, continuity corrections and approximate integer thresholds. Seven accessible diagrams show model areas, table reading, aligned scales, intervals, quantiles, matched binomial and normal shapes and whole integer cells. All five Statistics 1 sections now have teaching and staged practice linked to their mapped requirements. Fourteen Poisson distribution lessons cover all five official section 6.1 requirements, with homogeneous event-count assumptions, exposure scaling, exact masses and tails, conditional count events, moments, parameter inference, probability duration targets, disjoint and overlapping windows, rare-count binomial approximations, matched normal approximations and continuity corrections. Nine accessible diagrams show event exposures, unit conversions, discrete bars, complete tail complements, population moments, shared windows, approximate count models and integer cells. Exact checks distinguish approximate and exact close-target decisions. Fourteen Linear combinations lessons cover the six listed results in official section 6.2, with affine moments, signed scales and constant cases, expectation without independence, independent squared variance contributions, fixed and random fees, independent copies versus reused values, affine normal events, independent normal sums and differences, weighted and conditional areas, coefficient inference, exact independent Poisson sums and overlapping exposure limits. Nine accessible diagrams distinguish scale changes, variance contributions, component costs, copy mechanisms, aligned normal scales, summed areas, Poisson count pairs and unsupported distribution claims. Fourteen Continuous random variables lessons cover both official section 6.3 requirements, with density validity and units, support-aware intervals, complements and conditional regions, first and second moments, percentiles by direct density areas, symmetry checks, parameter inference, infinite-support power and exponential integrals, divergent-moment limits and combined applications. Nine accessible diagrams distinguish density height from probability, normalisation, negative-density failures, central and conditional areas, mean versus median, percentile locations and partial infinite tails. Explicit cumulative distribution function knowledge is not required. Eighteen Sampling and estimation lessons cover all eight official section 6.4 requirements, with population and frame definitions, random-number selection, sampling-quality limits, independent sample-mean moments, normal means, CLT conditions, SE scaling, raw, summarised and coded unbiased estimates, known-variance and large-sample mean intervals, repeated-sampling interpretation, precision planning, proportion intervals, sparse-outcome limits and contextual applications. Ten accessible diagrams distinguish selection frames, number rejection, repeated sample means, population versus mean density, CLT models, variance divisors, interval targets, illustrative coverage, integer size checks and proportion margins. Eighteen Hypothesis tests lessons cover all five official section 6.5 requirements, with pre-specified alternatives, inclusive count tails, explicit equal-tail conventions, integer critical regions and actual significance, exposure-aware Poisson tests, continuity-corrected count approximations, normal and large-sample mean tests, confidence-interval connections, error classifications, fixed-region binomial, Poisson and normal error probabilities and sample-size power comparisons. Nine accessible diagrams show the testing procedure, exact count regions, unequal Poisson tails, discrete significance jumps, corrected count boundaries, matching mean intervals and alternative error distributions. All shared lessons are counted once in the course total. All 38 sections now link teaching and practice. Eight earlier overview lessons remain available, but do not count as full section coverage. This is a content-availability measure, not full programme completion or exam readiness. Written practice is self-checked, not officially marked. Check the 2028–2030 syllabus separately if sitting an examination from 2028; this teaching map uses 2026–2027 requirements.

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