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Date May 2022 Marks available 3 Reference code 22M.1.SL.TZ1.8
Level Standard Level Paper Paper 1 Time zone Time zone 1
Command term Find Question number 8 Adapted from N/A

Question

A factory produces bags of sugar with a labelled weight of 500g. The weights of the bags are normally distributed with a mean of 500g and a standard deviation of 3g.

A bag that weighs less than 495g is rejected by the factory for being underweight.

Write down the percentage of bags that weigh more than 500g.

[1]
a.

Find the probability that a randomly chosen bag is rejected for being underweight.

[2]
b.

A bag that weighs more than k grams is rejected by the factory for being overweight. The factory rejects 2% of bags for being overweight.

Find the value of k.

[3]
c.

Markscheme

50%         A1


Note:
Do not accept 0.5 or 12.

 

[1 mark]

a.

0.0478  0.0477903, 4.78%          A2

 

[2 marks]

b.

PX<k=0.98   OR   PX>k=0.02         (M1)


Note: Award (M1) for a sketch with correct region identified.

 

506g   506.161          A2

 

[3 marks]

c.

Examiners report

Many candidates made no attempt at this question, suggesting poor preparation, although it has appeared frequently in past sessions.

Few candidates recognized that a normal distribution is symmetric.

 

a.

Of the few candidates who were able to interpret the question correctly and use their calculator to identify the numerical digits satisfying the probability requirements, a number were out by a factor of 10.

 

b.

Few correct answers were seen. Of those that tried to show a diagram, there was often not enough detail to award a method mark (e.g. the shaded area was numerically identified as 2%).

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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