A-LevelMathematicsProbability and statistics9709/53 · May/June 2020 · Q46 Marks

A fair four-sided spinner has edges numbered 1, 2, 2, 3. A fair three-sided spinner has edges numbered -2, -1, 1. Each spinner is spun and the number on the edge on which it comes to rest is noted. The random variable X is the sum of the two numbers that have been noted.

Mark scheme

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

[B1] Probability distribution table with correct scores with at least one probability
[B1] At least 4 probabilities correct
[B1] All probabilities correct
[B1] E(X) = (-1+0+3+4+6+4)/12 = 16/12 = 4/3
[M1] Var(X) = (1+0+3+8+18+16)/12 - (4/3)^2
[A1] 37/18 (= 2.06)

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Topic: Probability and statistics, A-Level Mathematics (syllabus 9709). Structured question, 6 marks.
Source: A-Level Mathematics, May/June 2020 Paper 5 Variant 3, question 4.

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