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A Level · Cambridge (CIE) · 9709

Define equiprobable outcomes before calculating probability

Count elementary outcomes rather than treating every summary label as equally likely.

Section 5.3 · Probability

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

  • List ordered outcomes systematically.
  • Simplify fractions and count successive choices.

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

Quick readiness check

For two distinguishable six-sided dice, how many ordered pairs exist?

What you will learn

  • Construct a complete finite sample space with stated equal-likelihood assumptions.
  • Calculate event probabilities and recognise non-equiprobable summaries.
A six-by-six ordered-pair grid for two fair independent dice shows the first result by row and the second by column. Each cell prints the sum. Six highlighted cells have total 7: (1,6), (2,5), (3,4), (4,3), (5,2), (6,1). All 36 elementary pairs are equally likely, giving probability 1/6; the printed total labels are not equiprobable.Count ordered pairs, not total labelsSecond dieFirst die1122334455662345673456784567895678910678910117891011126 highlighted pairs among 36P(total 7) = 6/36 = 1/6Fair independent dice assumed
A six-by-six ordered-pair grid for two fair independent dice shows the first result by row and the second by column. Each cell prints the sum. Six highlighted cells have total 7: (1,6), (2,5), (3,4), (4,3), (5,2), (6,1). All 36 elementary pairs are equally likely, giving probability 1/6; the printed total labels are not equiprobable.

Use the elementary experiment

Two fair independent six-sided dice give 36 equally likely ordered pairs (first die, second die). The pairs (1, 6) and (6, 1) are different elementary outcomes. A total of 7 occurs in six pairs, so its probability is 6/36 = 1/6. A total of 2 occurs only at (1, 1), with probability 1/36.

The eleven possible totals from 2 to 12 are not equally likely. Dividing by 11 counts summary labels and ignores how many elementary pairs create each label. The favourable-count divided by total-count rule requires an equiprobable sample space; if outcomes have different probabilities, add their actual weights instead.

Repeated labels do not erase elementary outcomes

Three independent fair coin tosses have eight equally likely ordered H/T strings. Exactly two heads occurs at HHT, HTH and THH, so its probability is 3/8. At least two heads includes HHH as well, giving 4/8 = 1/2. Counts of heads 0, 1, 2, 3 do not have equal probabilities.

A fair spinner with four equal sectors labelled 1, 1, 2, 3 has four equally likely sector outcomes. The label 1 has probability 2/4 = 1/2, while 2 and 3 each have probability 1/4. Fairness concerns equal sector areas under the stipulated spinner model, not equality of distinct printed labels. State assumptions instead of inferring fairness from appearance alone.

Pause and explain

What is P(total 7) for two fair independent dice?

Put the idea to work

Worked example

Two fair independent dice are rolled. Find the probability of total 7 and explain why counting totals equally fails.

Show the worked solution
  1. There are 6 × 6 = 36 equiprobable ordered pairs.
  2. Total 7 has pairs (1,6), (2,5), (3,4), (4,3), (5,2), (6,1).
  3. Its probability is 6/36 = 1/6; totals have different numbers of contributing pairs.

Answer P(total 7) = 1/6; total labels are not equiprobable.

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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

Find P(total 7) when two fair independent six-sided dice are rolled.

Give me a hint

List all ordered pairs with the required total.

Compare my reasoning
  1. The sample space contains 36 equiprobable ordered pairs.
  2. The six favourable pairs are (1,6), (2,5), (3,4), (4,3), (5,2), (6,1).
  3. Divide 6 by 36 and simplify to 1/6.

1/6.

Look for these in your work

  • Use the ordered-pair sample space.
  • State the fair independent dice assumptions.
2 · Independent

Three independent fair coins are tossed. Find the probability of at least two heads.

Give me a hint

Include exactly two heads and the all-heads string.

Compare my reasoning
  1. Eight ordered H/T strings are equiprobable.
  2. The favourable strings are HHT, HTH, THH and HHH.
  3. Their probability is 4/8 = 1/2.

1/2.

Look for these in your work

  • Interpret at least two correctly.
  • Do not give equal weight to different head counts.
3 · Transfer

A fair spinner has four equal sectors labelled 1, 1, 2, 3. Find P(label 1) and explain why 1/3 is wrong.

Give me a hint

The equal sectors, not the distinct labels, are the elementary outcomes.

Compare my reasoning
  1. All four sectors have probability 1/4 in the stated model.
  2. Two sectors carry label 1, so its probability is 2/4 = 1/2.
  3. The value 1/3 incorrectly treats the three distinct labels as equiprobable.

1/2; repeated label 1 occupies two of four equal sectors.

Look for these in your work

  • Retain both sectors with the repeated label.
  • Identify exactly what is equally likely.

Common mistakes

  • Keep the order of distinguishable dice or tosses.
  • Count elementary outcomes, not equally weighted summary labels.
  • State fairness and independence when the model needs them.
Recall without your notes

What can you explain now?

Are the four possible head counts for three fair coin tosses equally likely?

Compare with the explanation

No; their elementary-outcome counts are 1, 3, 3 and 1.

The eight ordered toss strings are equiprobable, not the four head-count labels.

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Make it stick

Choose the elementary outcomes and justify equal likelihood before using favourable count divided by total count.

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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