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Date May 2018 Marks available 7 Reference code 18M.2.AHL.TZ1.H_8
Level Additional Higher Level Paper Paper 2 Time zone Time zone 1
Command term Find Question number H_8 Adapted from N/A

Question

Each of the 25 students in a class are asked how many pets they own. Two students own three pets and no students own more than three pets. The mean and standard deviation of the number of pets owned by students in the class are  18 25 and  24 25  respectively.

Find the number of students in the class who do not own a pet.

Markscheme

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

METHOD 1

let p have no pets, q have one pet and r have two pets      (M1)

p + q + r + 2 = 25      (A1)

0p + 1q + 2r + 6 = 18      A1

Note: Accept a statement that there are a total of 12 pets.

attempt to use variance equation, or evidence of trial and error       (M1)

0 p + 1 q + 4 r + 18 25 ( 18 25 ) 2 = ( 24 25 ) 2      (A1)

attempt to solve a system of linear equations (M1)

p = 14      A1

 

METHOD 2

     (M1)

p + q + r + 2 25 = 1       (A1)

q + 2 r + 6 25 = 18 25 ( q + 2 r = 12 25 )       A1

q + 4 r + 18 25 ( 18 25 ) 2 = 576 625 ( q + 4 r = 18 25 )       (M1)(A1)

q = 6 25 , r = 3 25       (M1)

p = 14 25        A1

so 14 have no pets

[7 marks]

Examiners report

[N/A]

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

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