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A Level · Cambridge (CIE) · 9709

Follow probability trees without replacement

Update every second-stage branch after removal, multiply along a path and add distinct paths for an event.

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 fractions and conditional probabilities.
  • Use combinations for random subsets.

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

Quick readiness check

After removing one red from four red and three blue, what remains?

What you will learn

  • Construct a without-replacement tree with changing conditional probabilities.
  • Combine ordered paths and check against unordered selection counts.
A two-draw tree without replacement starts with four red and three blue. First R and B branches are 4/7 and 3/7. After R, second R and B are 3/6 and 3/6; after B they are 4/6 and 2/6. Joint leaves RR, RB, BR, BB have probabilities 2/7, 2/7, 2/7, 1/7. They sum to one, and different colours combine RB and BR to give 4/7.Update the remaining bagStart4/7R3/6RR 2/73/6RB 2/73/7B4/6BR 2/72/6BB 1/7Different colours: 2/7 + 2/7 = 4/7All four joint leaves sum to 1
A two-draw tree without replacement starts with four red and three blue. First R and B branches are 4/7 and 3/7. After R, second R and B are 3/6 and 3/6; after B they are 4/6 and 2/6. Joint leaves RR, RB, BR, BB have probabilities 2/7, 2/7, 2/7, 1/7. They sum to one, and different colours combine RB and BR to give 4/7.

Every branch refers to its history

A bag contains four red and three blue balls, all individually equally likely to be drawn. Draw two without replacement. First branches are R with 4/7 and B with 3/7. After R, the bag has three red and three blue, so the second branches are 3/6 and 3/6. After B, they are 4/6 and 2/6.

At each node the outgoing probabilities sum to one. Multiply along a path: P(RR) = (4/7)(3/6) = 2/7, P(RB) = 2/7, P(BR) = 2/7 and P(BB) = 1/7. These joint path probabilities add to one. The second-stage branch after R is P(second R | first R), not the unconditional initial red probability.

An event may contain several paths

Different colours means RB or BR, two disjoint paths. Add their joint probabilities to obtain 4/7. Do not use only one order unless the event specifically requests that order. The combination check gives 4 × 3/7C2 = 12/21 = 4/7.

For three draws from five red and three blue without replacement, exactly two red means RRB, RBR or BRR. Each path has probability 5/28, so the total is 15/28. The order-independent subset count gives 5C2 × 3C1/8C3 = 15/28. Use the changing numerator and denominator at each draw, including the third stage.

Pause and explain

Four red, three blue; two draws without replacement. What is P(RR)?

Put the idea to work

Worked example

Draw twice without replacement from four red and three blue. Find P(different colours).

Show the worked solution
  1. P(RB) = (4/7)(3/6) = 2/7.
  2. P(BR) = (3/7)(4/6) = 2/7.
  3. Add the two mutually exclusive paths to get 4/7.

Answer 4/7.

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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

Draw twice without replacement from four red and three blue. Find P(both red).

Give me a hint

The second red branch follows a first-red removal.

Compare my reasoning
  1. First red has probability 4/7.
  2. After red, three of six remaining balls are red, giving 3/6.
  3. Multiply to obtain (4/7)(3/6) = 2/7.

2/7.

Look for these in your work

  • Use the changing bag contents.
  • Multiply successive conditional branch probabilities.
2 · Independent

For that bag and two draws without replacement, find P(different colours).

Give me a hint

The event has two possible colour orders.

Compare my reasoning
  1. P(RB) = (4/7)(3/6) = 2/7.
  2. P(BR) = (3/7)(4/6) = 2/7.
  3. The paths are disjoint, so their sum is 4/7.

4/7.

Look for these in your work

  • Include both colour orders.
  • Distinguish path multiplication from event addition.
3 · Transfer

Draw three times without replacement from five red and three blue. Find P(exactly two red) using ordered paths, then check with combinations.

Give me a hint

The favourable paths are RRB, RBR and BRR.

Compare my reasoning
  1. Each path has numerator product 5 × 4 × 3 and denominator 8 × 7 × 6, giving 5/28.
  2. The three disjoint paths total 15/28.
  3. The subset check gives 5C2 × 3C1/8C3 = 30/56 = 15/28.

15/28 by both methods.

Look for these in your work

  • Update all three stages without replacement.
  • List every order belonging to the exact composition.

Common mistakes

  • Reduce the total after every removal.
  • Change the colour count only when that colour was removed.
  • Add all permitted orders after multiplying each path.
Recall without your notes

What can you explain now?

Is the second red branch after a first red still 4/7?

Compare with the explanation

No; it is 3/6 because one red was removed from the seven-ball bag.

Without replacement changes both the relevant remaining count and total.

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Make it stick

Write the remaining bag at each node, multiply down paths, and add the distinct paths belonging to the event.

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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