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Date November 2021 Marks available 2 Reference code 21N.1.AHL.TZ0.14
Level Additional Higher Level Paper Paper 1 Time zone Time zone 0
Command term Find Question number 14 Adapted from N/A

Question

On Paul’s farm, potatoes are packed in sacks labelled 50kg. The weights of the sacks of potatoes can be modelled by a normal distribution with mean weight 49.8kg and standard deviation 0.9kg.

Find the probability that a sack is under its labelled weight.

[2]
a.

Find the lower quartile of the weights of the sacks of potatoes.

[2]
b.

The sacks of potatoes are transported in crates. There are 10 sacks in each crate and the weights of the sacks of potatoes are independent of each other.

Find the probability that the total weight of the sacks of potatoes in a crate exceeds 500kg.

[3]
c.

Markscheme

let X be the random variable “the weight of a sack of potatoes”

PX<50                 (M1)

=0.588kg   0.587929                 A1

 

[2 marks]

a.

PX<l=0.25                 (M1)

49.2kg   49.1929                 A1

 

[2 marks]

b.

attempt to sum 10 independent random variables                 (M1)

Y=Σi=110Xi~N498, 10×0.92                 (A1)

PY>500=0.241                 A1

 

[3 marks]

c.

Examiners report

The first part of the question was often answered well but there were a number of candidates who interpreted finding PX<50 by finding PX<49.9 or something similar. Not all candidates, however, understood that the lower quartile is given by PX<l=0.25. Part (c) was less well understood. Attempts to sum 10 independent random variables correctly involved multiplication of the mean by 10 but the standard deviation and not the variance was incorrectly multiplied by 10.

a.

The first part of the question was often answered well but there were a number of candidates who interpreted finding PX<50 by finding PX<49.9 or something similar. Not all candidates, however, understood that the lower quartile is given by PX<l=0.25. Part (c) was less well understood. Attempts to sum 10 independent random variables correctly involved multiplication of the mean by 10 but the standard deviation and not the variance was incorrectly multiplied by 10.

b.

The first part of the question was often answered well but there were a number of candidates who interpreted finding PX<50 by finding PX<49.9 or something similar. Not all candidates, however, understood that the lower quartile is given by PX<l=0.25. Part (c) was less well understood. Attempts to sum 10 independent random variables correctly involved multiplication of the mean by 10 but the standard deviation and not the variance was incorrectly multiplied by 10.

c.

Syllabus sections

Topic 4—Statistics and probability » SL 4.3—Mean, median, mode. Mean of grouped data, standard deviation. Quartiles, IQR
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