A population is normally distributed with mean 35 and standard deviation 8.1 . A random sample of size 140 is chosen from this population and the sample mean is denoted by X.
- (a)Find P(X> 36).[3]
- (b)It is given that P(X < a) = 0.986 . Find the value of a.[3]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.
- (36-35) / (8.1÷√140) [= 1.461]
- 1 – Ф(‘1.461’)
- = 0.0720 (3 sf)
- [Φ⁻¹(0.986)] = 2.197 to 2.198
- (a-35) / (8.1÷√140) = ± '2.198'
- a = 36.5 (3 sf)
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