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Date May 2017 Marks available 3 Reference code 17M.2.AHL.TZ2.H_3
Level Additional Higher Level Paper Paper 2 Time zone Time zone 2
Command term Calculate Question number H_3 Adapted from N/A

Question

Packets of biscuits are produced by a machine. The weights X , in grams, of packets of biscuits can be modelled by a normal distribution where X N ( μ ,   σ 2 ) . A packet of biscuits is considered to be underweight if it weighs less than 250 grams.

The manufacturer makes the decision that the probability that a packet is underweight should be 0.002. To do this μ is increased and σ remains unchanged.

The manufacturer is happy with the decision that the probability that a packet is underweight should be 0.002, but is unhappy with the way in which this was achieved. The machine is now adjusted to reduce σ and return μ to 253.

Given that μ = 253 and σ = 1.5 find the probability that a randomly chosen packet of biscuits is underweight.

[2]
a.

Calculate the new value of μ giving your answer correct to two decimal places.

[3]
b.

Calculate the new value of σ .

[2]
c.

Markscheme

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

P ( X < 250 ) = 0.0228      (M1)A1

[2 marks]

a.

250 μ 1.5 = 2.878      (M1)(A1)

μ = 254.32      A1

 

Notes:     Only award A1 here if the correct 2dp answer is seen. Award M0 for use of 1.5 2 .

 

[3 marks]

b.

250 253 σ = 2.878      (A1)

σ = 1.04      A1

[2 marks]

c.

Examiners report

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

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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