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Date November 2021 Marks available 4 Reference code 21N.2.SL.TZ0.4
Level Standard Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number 4 Adapted from N/A

Question

A factory manufactures lamps. It is known that the probability that a lamp is found to be defective is 0.05. A random sample of 30 lamps is tested.

Find the probability that there is at least one defective lamp in the sample.

[3]
a.

Given that there is at least one defective lamp in the sample, find the probability that there are at most two defective lamps.

[4]
b.

Markscheme

recognize that the variable has a Binomial distribution              (M1)

X~B30,0.05

attempt to find PX1              (M1)

1-PX=0  OR  1-0.9530  OR  1-0.214638  OR  0.785361


Note: The two M marks are independent of each other.


PX1=0.785                  A1

 

[3 marks]

a.

recognition of conditional probability             (M1)

PX2X1  OR  Pat most 2 defective|at least 1 defective


Note: Recognition must be shown in context either in words or symbols but not just PAB.


P1X2PX1  OR  PX=1+PX=2PX1              (A1)

0.5975400.785361  OR  0.812178-0.2146380.785361  OR  0.338903+0.2586360.785361              (A1)

=0.760847

PX2X1=0.761                 A1

 

[4 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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Topic 4—Statistics and probability » SL 4.8—Binomial distribution
Topic 4—Statistics and probability

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