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Date May 2022 Marks available 2 Reference code 22M.2.SL.TZ1.6
Level Standard Level Paper Paper 2 Time zone Time zone 1
Command term Find Question number 6 Adapted from N/A

Question

Let A and B be two independent events such that P(AB')=0.16 and P(AB)=0.36.

Given that P(AB)=x, find the value of x.

[4]
a.

Find PA'B'.

[2]
b.

Markscheme

METHOD 1

EITHER

one of PA=x+0.16  OR  PB=x+0.36           A1


OR

           A1

 

THEN

attempt to equate their P(AB) with their expression for PA×PB           M1

P(AB)=PA×PBx=x+0.16×x+0.36           A1

x=0.24           A1

 

METHOD 2

attempt to form at least one equation in PA and PB using independence           M1

PAB'=PA×PB' PA×1-PB=0.16  OR

PA'B=PA'×PB 1-PA×PB=0.36

PA=0.4 and PB=0.6           A1

P(AB)=PA×PB=0.4×0.6            (A1)

x=0.24           A1

 

[4 marks]

a.

METHOD 1

recognising PA'B'=PA'            (M1)

=1-0.16-0.24

=0.6           A1

 

METHOD 2

PB=0.36+0.24=0.6

PA'B'=PA'B'PB'  =0.240.4            (A1)

=0.6           A1

 

[2 marks]

b.

Examiners report

[N/A]
a.
[N/A]
b.

Syllabus sections

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