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Date May 2022 Marks available 7 Reference code 22M.2.AHL.TZ2.8
Level Additional Higher Level Paper Paper 2 Time zone Time zone 2
Command term Find Question number 8 Adapted from N/A

Question

Rachel and Sophia are competing in a javelin-throwing competition.

The distances, R metres, thrown by Rachel can be modelled by a normal distribution with mean 56.5 and standard deviation 3.

The distances, S metres, thrown by Sophia can be modelled by a normal distribution with mean 57.5 and standard deviation 1.8.

In the first round of competition, each competitor must have five throws. To qualify for the next round of competition, a competitor must record at least one throw of 60 metres or greater in the first round.

Find the probability that only one of Rachel or Sophia qualifies for the next round of competition.

Markscheme

Rachel: R~N56.5,32

PR60=0.1216          (A1)

Sophia: S~N57.5,1.82

PS60=0.0824          (A1)

recognises binomial distribution with n=5          (M1)

let NR represent the number of Rachel’s throws that are longer than 60 metres

NR~B5,0.1216

either PNR1=0.4772  or  PNR=0=0.5227          (A1)

let NS represent the number of Sophia’s throws that are longer than 60 metres

NS~B5,0.0824

either PNS1=0.3495  or  PNS=0=0.6504          (A1)


EITHER

uses PNR1PNS=0+PNS1PNR=0          (M1)

Pone of Rachel or Sophia qualify=0.4772×0.6504+0.3495×0.5227


OR

uses PNR1+PNS1-2×PNR1×PNS1          (M1)

Pone of Rachel or Sophia qualify=0.4772+0.3495-2×0.4772×0.3495

 

THEN

=0.4931

 =0.493        A1


Note:
M marks are not dependent on the previous A marks.

 

[7 marks]

Examiners report

In general, well answered with many candidates getting full marks. On the other end of the spectrum, some candidates could only get the first two marks for normal distribution without recognising the binomial distribution and often just stopping there. Some candidates lost the last two marks for not being able to combine the probabilities correctly.

Syllabus sections

Topic 4—Statistics and probability » SL 4.6—Combined, mutually exclusive, conditional, independence, prob diagrams
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