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Date November 2016 Marks available 6 Reference code 16N.2.AHL.TZ0.H_8
Level Additional Higher Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number H_8 Adapted from N/A

Question

A random variable X is normally distributed with mean μ  and standard deviation σ , such that P ( X < 30.31 ) = 0.1180 and P ( X > 42.52 ) = 0.3060 .

Find μ and σ .

[6]
a.

Find P ( | X μ | < 1.2 σ ) .

[2]
b.

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

P ( X < 42.52 ) = 0.6940    (M1)

either P ( Z < 30.31 μ σ ) = 0.1180  or P ( Z < 42.52 μ σ ) = 0.6940      (M1)

30.31 μ σ = Φ 1 ( 0.1180 ) 1.1850    (A1)

42.52 μ σ = Φ 1 ( 0.6940 ) 0.5072    (A1)

attempting to solve simultaneously     (M1)

μ = 38.9 and σ = 7.22      A1

[6 marks]

a.

P ( μ 1.2 σ < X < μ + 1.2 σ ) (or equivalent eg. 2 P ( μ < X < μ + 1.2 σ ) )     (M1)

= 0.770    A1

 

Note: Award (M1)A1 for P ( 1.2 < Z < 1.2 ) = 0.770 .

[2 marks]

b.

Examiners report

[N/A]
a.
[N/A]
b.

Syllabus sections

Topic 4—Statistics and probability » SL 4.9—Normal distribution and calculations
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Topic 4—Statistics and probability

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