A-LevelMathematicsProbability and statistics9709/51 · Oct/Nov 2020 · Q46 Marks

The random variable X takes each of the values 1, 2, 3, 4 with probability ¹/₄. Two independent values of X are chosen at random. If the two values of X are the same, the random variable Y takes that value. Otherwise, the value of Y is the larger value of X minus the smaller value of X.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[B1] Probability distribution table with correct scores with at least one probability, allow extra score values if probability of zero stated'
[B1] One probability (linked with correct score) correct
[B1] 2 more probs (linked with correct scores) correct
[B1FT] 4th prob correct, FT sum of 3 or 4 terms = 1
[M1] their P(2) / (their P(2) + their P(4)) seen or correct outcome space.
[A1] 5/6 or 0.833

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Topic: Probability and statistics, A-Level Mathematics (syllabus 9709). Structured question, 6 marks.
Source: A-Level Mathematics, Oct/Nov 2020 Paper 5 Variant 1, question 4.

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