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Date November 2021 Marks available 5 Reference code 21N.2.SL.TZ0.9
Level Standard Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number 9 Adapted from N/A

Question

The random variable X follows a normal distribution with mean μ and standard deviation σ.

The avocados grown on a farm have weights, in grams, that are normally distributed with mean μ and standard deviation σ. Avocados are categorized as small, medium, large or premium, according to their weight. The following table shows the probability an avocado grown on the farm is classified as small, medium, large or premium.

The maximum weight of a small avocado is 106.2 grams.

The minimum weight of a premium avocado is 182.6 grams.

A supermarket purchases all the avocados from the farm that weigh more than 106.2 grams.

Find the probability that an avocado chosen at random from this purchase is categorized as

Find Pμ-1.5σ<X<μ+1.5σ.

[3]
a.

Find the value of μ and of σ.

[5]
b.

medium.

[2]
c.i.

large.

[1]
c.ii.

premium.

[1]
c.iii.

The selling prices of the different categories of avocado at this supermarket are shown in the following table:

 

 

 

 

The supermarket pays the farm $200 for the avocados and assumes it will then sell them in exactly the same proportion as purchased from the farm.

According to this model, find the minimum number of avocados that must be sold so that the net profit for the supermarket is at least $438.

[4]
d.

Markscheme

Pμ-1.5σ-μσ<X-μσ<μ+1.5σ-μσ                (M1)

P-1.5<Z<1.5  OR  1-2×PZ<-1.5                (A1)

P-1.5<Z<1.5=0.866385

Pμ-1.5σ<X<μ+1.5σ=0.866                A1


Note: Do not award any marks for use of their answers from part (b).

 

[3 marks]

a.

z1=-1.75068  and  z2=1.30468 (seen anywhere)                (A1)

correct equations                (A1)(A1)

106.2-μσ=-1.75068,  μ+1.30468σ=182.6

attempt to solve their equations involving z values                (M1)

μ=149.976..., σ=25.0051

μ=150, σ=25.0                A1

 

[5 marks]

b.

new sample space is 96% (may be seen in (ii) or (iii))                (M1)

Pmedium|not small  OR  0.5760.96

PMedium=0.6                A1

 

[2 marks]

c.i.

PLarge=0.3                A1

 

[1 mark]

c.ii.

PPremium=0.1                A1

 

[1 mark]

c.iii.

attempt to express revenue from avocados                (M1)

1.1×0.6+1.29×0.3+1.96×0.1  OR  1.243n

correct inequality or equation for net profit in terms of n                (A1)

1.1×0.6n+1.29×0.3n+1.96×0.1n-200438  OR  1.243n-200=438

attempt to solve the inequality                (M1)

sketch  OR  n=513.274...

n=514                A1


Note:
Only award follow through in part (d) for 3 probabilities which add up to 1. FT of probabilities from c) that do not add up to 1 should only be awarded M marks, where appropriate, in d).

 

[4 marks]

d.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.i.
[N/A]
c.ii.
[N/A]
c.iii.
[N/A]
d.

Syllabus sections

Topic 4—Statistics and probability » SL 4.6—Combined, mutually exclusive, conditional, independence, prob diagrams
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Topic 4—Statistics and probability » SL 4.7—Discrete random variables
Topic 4—Statistics and probability » SL 4.12—Z values, inverse normal to find mean and standard deviation
Topic 4—Statistics and probability

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