A-LevelMathematicsProbability and statistics9709/62 · May/June 2025 · Q34 Marks
The random variable X has the distribution Po(15). It is given that P(X = n) = P(X = n+1).
- (a)Write down an expression in terms of e for P(X = 12).[1]
- (b)Write down an equation in n, and hence find the value of n.[3]
Mark scheme
Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.
- [B1] e-15 x 1512 / 12! Seen.
- [B1] e-15 x 15n / n! = e-15 x 15n+1 / (n+1)! If brackets (n+1)! missing allow benefit of doubt.
- [M1] OE. Attempt to legitimately remove powers and factorials i.e. powers reduced to 15/n seen, and factorials reduced to n + 1 seen.
- [A1] n = 14 Note: Trial and error solutions: B1B2 for 14.
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