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A Level · Cambridge (CIE) · 9709

Use selections and ordered assignments to calculate probability

Apply counting methods to favourable and total outcomes using the same sampling model.

Statistics 1 · Probability · 5.3
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

  • Calculate permutations and combinations.
  • Distinguish membership from named roles.

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

Quick readiness check

Which count describes unordered three-ball samples from eight distinct balls?

What you will learn

  • Use combinations for equally likely unordered samples.
  • Use permutations for ordered assignments without mixing outcome conventions.
Five distinct red balls R1–R5 and three distinct blue balls B1–B3 form eight individuals. Uniformly selecting three without replacement gives 8C3 = 56 equally likely unordered subsets. Exactly two red and one blue gives 5C2 × 3C1 = 30 favourable subsets, so probability 15/28. Colour compositions are not equally likely labels.Uniform three-ball subsetsR1R2R3R4R5B1B2B3Favourable: 5C2 × 3C1 = 30All subsets: 8C3 = 56P(exactly two red) = 15/28Each physical ball remains distinct
Five distinct red balls R1–R5 and three distinct blue balls B1–B3 form eight individuals. Uniformly selecting three without replacement gives 8C3 = 56 equally likely unordered subsets. Exactly two red and one blue gives 5C2 × 3C1 = 30 favourable subsets, so probability 15/28. Colour compositions are not equally likely labels.

A random subset uses combinations

A bag has four distinct blue and three distinct red balls. Choose three without replacement, with every three-ball subset equally likely. There are 7C3 = 35 total subsets. Exactly two blue and one red gives 4C2 × 3C1 = 18 favourable subsets, so probability 18/35. Same-colour balls remain distinct physical individuals when forming the sample space.

For five red and three blue balls, a random three-ball subset has 8C3 = 56 possibilities. Exactly two red and one blue gives 5C2 × 3C1 = 30, hence 30/56 = 15/28. Do not treat colour-composition labels as equally likely outcomes; they can represent very different numbers of subsets.

An assignment uses ordered outcomes

Six equally likely no-tie competitors fill first, second and third places. The model has 6P3 = 120 equally likely podiums. If A and B must both appear, choose the third competitor from the other four, then arrange the three in 3! ways: 24 favourable podiums. Their probability is 24/120 = 1/5.

A matching unordered method chooses three podium members: favourable sets 4 and all sets 6C3 = 20, also giving 1/5, because all six orders within each set have the same probability in this model. Dividing an unordered favourable count by an ordered total would be inconsistent. Equal podium probabilities are a stipulated mathematical model, not a claim about competitors’ real abilities.

Pause and explain

Five red and three blue: what is P(exactly two red in a random three-ball subset)?

Put the idea to work

Worked example

Choose three balls at random without replacement from five distinct red and three distinct blue balls. Find P(exactly two red).

Show the worked solution
  1. All unordered samples number 8C3 = 56.
  2. Choose two red and one blue in 5C2 × 3C1 = 30 ways.
  3. The probability is 30/56 = 15/28 under uniform subset sampling.

Answer 15/28.

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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

Select two books uniformly without replacement from two distinct fiction and three distinct non-fiction books. Find P(both fiction).

Give me a hint

There is one favourable fiction pair among all two-book subsets.

Compare my reasoning
  1. The total subset count is 5C2 = 10.
  2. Both fiction gives 2C2 = 1 favourable pair.
  3. Divide 1 by 10 to get 1/10.

1/10.

Look for these in your work

  • Use a uniform unordered-pair model.
  • Keep favourable and total counts consistent.
2 · Independent

Select three balls uniformly without replacement from five distinct red and three distinct blue balls. Find P(exactly two red).

Give me a hint

Choose two red individuals and one blue individual.

Compare my reasoning
  1. All subsets number 8C3 = 56.
  2. Favourable subsets number 5C2 × 3C1 = 30.
  3. The probability is 30/56 = 15/28.

15/28.

Look for these in your work

  • Apply the exact composition.
  • Simplify the ratio after counting.
3 · Transfer

In a stipulated model with 120 equally likely no-tie podiums from six competitors A–F, find P(A and B both appear). Check with an unordered method.

Give me a hint

Choose the third member, then assign the three places.

Compare my reasoning
  1. Four third-member choices and six place orders give 24 favourable podiums.
  2. Their probability is 24/120 = 1/5.
  3. Unordered membership gives 4/6C3 = 4/20 = 1/5 as well.

1/5 by both consistent methods.

Look for these in your work

  • Retain both required people.
  • Use a consistent outcome convention in each ratio.

Common mistakes

  • Use the same ordered or unordered convention in both counts.
  • Treat each physical ball as an individual in the sample space.
  • Equal likelihood belongs to the stated model, not a real-world ranking assumption.
Recall without your notes

What can you explain now?

Can an unordered favourable count be divided by an ordered total?

Compare with the explanation

No; numerator and denominator must count the same kind of outcome.

Otherwise one count includes extra orderings that the other omits.

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

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Make it stick

Count favourable and total outcomes under one sampling model; use sets for membership and ordered assignments for roles.

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.

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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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