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Date May 2019 Marks available 2 Reference code 19M.1.AHL.TZ1.H_6
Level Additional Higher Level Paper Paper 1 Time zone Time zone 1
Command term Find Question number H_6 Adapted from N/A

Question

Let X be a random variable which follows a normal distribution with mean μ. Given that P(X<μ5)=0.2 , find

P(X>μ+5).

[2]
a.

P(X<μ+5|X>μ5).

[5]
b.

Markscheme

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

use of symmetry eg diagram       (M1)

P(X>μ+5)=0.2       A1

[2 marks]

a.

EITHER

P(X<μ+5|X>μ5)=P(X>μ5X<μ+5)P(X>μ5)       (M1)

      =P(μ5<X<μ+5)P(X>μ5)       (A1)

      =0.60.8      A1A1

Note: A1 for denominator is independent of the previous A marks.

OR

use of diagram       (M1)

Note: Only award (M1) if the region μ5<X<μ+5 is indicated and used.

P(X>μ5)=0.8      P(μ5<X<μ+5)=0.6       (A1)

Note: Probabilities can be shown on the diagram.

=0.60.8      M1A1

THEN

=34=(0.75)      A1

[5 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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