A fair spinner has edges numbered 0, 1, 2, 2. Another fair spinner has edges numbered –1, 0, 1. Each spinner is spun. The number on the edge on which a spinner comes to rest is noted. The random variable X is the sum of the numbers for the two spinners.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

1. [B1] Table with x values and at least one probability substituted, 0 <p<1. Condone any additional x values if probability stated as 0.
2. [B1] 2 correct identified probabilities.
3. [B1] All probabilities correct (accept to 3sf). SC if less than 2 correct probabilities: SC B1 4 or 5 probabilities summing to one.
4. [M1] E(X) = [-1/12 + 0/12 + 6/12 + 6/12 + 15/12] May be implied by use in Variance, accept unsimplified expression. Probabilities must sum to 1 ± 0.001.
5. [M1] Appropriate variance formula using their (E(X))^2. FT accept probabilities not summing to 1.
6. [A1] WWW