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A Level · Cambridge (CIE) · 9709

Solve unknown probability parameters and mixed conditions

Use model equations, restrict the conditional universe and reject parameter values outside the physical assumptions.

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

  • Solve a quadratic and check admissible roots.
  • Use combinations, conditional ratios and probability trees.

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

Quick readiness check

Which value can represent a number of red balls?

What you will learn

  • Infer an unknown count or source weight from a supplied probability.
  • Combine selection restrictions with conditioning and verify the result.

An unknown count must remain a valid integer

A bag has r red and two blue balls. Draw two uniformly without replacement; suppose P(both red) = 2/5. Then r(r − 1)/[(r + 2)(r + 1)] = 2/5. Rearranging gives 3r² − 11r − 4 = 0 = (3r + 1)(r − 4), so r = 4 or r = −1/3. Only the non-negative integer count r = 4 fits the bag model.

Substitute back: (4/6)(3/5) = 2/5. A probability equation can have algebraic solutions that violate count, positivity or denominator conditions. Always verify the chosen value in the original experiment rather than relying only on a rearranged equation.

Conditioning and selection restrictions change different parts

Choose three people uniformly from five A-group and three B-group candidates in disjoint groups. Given at least one B was selected, all-A teams are removed, leaving 8C3 − 5C3 = 46 equally likely eligible subsets. Exactly two B has 3C2 × 5C1 = 15 favourable subsets, so the conditional probability is 15/46, not the unconditional 15/56.

Choose box A with unknown probability q and B with 1 − q; their red probabilities are 3/4 and 1/4. If total red probability is 1/2, then (3/4)q + (1/4)(1 − q) = 1/2, giving q = 1/2. Given red, the A probability is [(1/2)(3/4)]/(1/2) = 3/4. The source weight and reverse conditional are distinct quantities, both within [0,1].

Pause and explain

Five A and three B candidates; choose three, given at least one B. What is P(exactly two B)?

Put the idea to work

Worked example

A bag has r red and two blue; two draws without replacement have P(both red) = 2/5. Find r.

Show the worked solution
  1. Form r(r − 1)/[(r + 2)(r + 1)] = 2/5.
  2. Rearrange to (3r + 1)(r − 4) = 0, giving 4 or −1/3.
  3. Keep integer r = 4 and check (4/6)(3/5) = 2/5.

Answer r = 4.

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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

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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

A bag has r red and two blue. Two uniform draws without replacement have P(both red) = 2/5. Find r and verify it.

Give me a hint

Write the sequential fraction, then reject roots that cannot be individual counts.

Compare my reasoning
  1. Set r(r − 1)/[(r + 2)(r + 1)] = 2/5.
  2. Rearrange to (3r + 1)(r − 4) = 0; valid count r = 4.
  3. Check (4/6)(3/5) = 2/5 in the original model.

r = 4; the root −1/3 is inadmissible.

Look for these in your work

  • Show the probability equation.
  • Check integer and original-denominator conditions.
2 · Independent

Choose three uniformly from five A and three B candidates in disjoint groups. Given at least one B, find P(exactly two B).

Give me a hint

Remove all-A teams from the conditional total.

Compare my reasoning
  1. All subsets number 56; all-A subsets number 10, leaving 46 eligible teams.
  2. Exactly two B gives 3C2 × 5C1 = 15 subsets.
  3. The conditional probability is 15/46.

15/46.

Look for these in your work

  • Apply the given information to the denominator.
  • Count exactly two B without ordering team members.
3 · Transfer

Choose box A with probability q or B with 1 − q. Red probabilities are 3/4 in A and 1/4 in B. Total P(red) is 1/2. Find q, then P(A|red).

Give me a hint

First solve the weighted total, then use a joint-over-observed-total ratio.

Compare my reasoning
  1. (3/4)q + (1/4)(1 − q) = 1/2 gives q = 1/2.
  2. A-and-red is (1/2)(3/4) = 3/8.
  3. P(A|red) = (3/8)/(1/2) = 3/4; both values meet the probability bounds.

q = 1/2; P(A|red) = 3/4.

Look for these in your work

  • Separate the unknown source weight from the reverse condition.
  • Verify the total and admissible probability range.

Common mistakes

  • Reject non-integer or negative ball counts.
  • Use a conditional denominator after excluding incompatible subsets.
  • Check source weights and probabilities against their [0,1] domains.
Recall without your notes

What can you explain now?

Why is 15/56 not the conditional team probability here?

Compare with the explanation

It includes ten all-A teams that contradict the given at-least-one-B information.

The conditioned total is 46; the favourable fifteen teams remain eligible.

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

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Make it stick

Build the model equation, restrict the conditional universe, and verify every inferred parameter in the original experiment.

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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