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Date November 2018 Marks available 3 Reference code 18N.2.SL.TZ0.S_9
Level Standard Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number S_9 Adapted from N/A

Question

A nationwide study on reaction time is conducted on participants in two age groups. The participants in Group X are less than 40 years old. Their reaction times are normally distributed with mean 0.489 seconds and standard deviation 0.07 seconds.

The participants in Group Y are 40 years or older. Their reaction times are normally distributed with mean 0.592 seconds and standard deviation σ seconds.

In the study, 38 % of the participants are in Group X.

A randomly selected participant has a reaction time greater than 0.65 seconds. Find the probability that the participant is in Group X.

[6]
c.

Ten of the participants with reaction times greater than 0.65 are selected at random. Find the probability that at least two of them are in Group X.

[3]
d.

Markscheme

correct work for P(group X and t > 0.65) or P(group Y and t  > 0.65)  (may be seen anywhere)     (A1)

eg     P ( group X ) × P ( t > 0.65 | X ) ,    P ( X t > 0.65 ) = 0.0107 × 0.38 ( = 0.004075 ) ,

P ( Y t > 0.65 ) = 0.396 × 0.62

recognizing conditional probability (seen anywhere)      (M1)

eg     P ( X | t > 0.65 ) ,    P ( A | B ) = P ( A B ) P ( B )

valid approach to find  P ( t > 0.65 )      (M1)

eg   ,   P ( X and  t > 0.65 ) + P ( Y and  t > 0.65 )

correct work for  P ( t > 0.65 )      (A1)

eg   0.0107 × 0.38 + 0.396 × 0.62,  0.249595

correct substitution into conditional probability formula      A1

eg    0.0107 × 0.38 0.0107 × 0.38 + 0.396 × 0.62 ,   0.004075 0.249595

0.016327

P ( X | t > 0.65 ) = 0.0163270      A1 N3

 

[6 marks]

c.

recognizing binomial probability      (M1)

eg     X B ( n , p ) ,   ( n r ) p r q n r ,  (0.016327)2(0.983672)8,   ( 10 2 )

valid approach      (M1)

eg    P ( X 2 ) = 1 P ( X 1 ) ,    1 P ( X < a ) ,  summing terms from 2 to 10 (accept binomcdf(10, 0.0163, 2, 10))

0.010994

P ( X 2 ) = 0.0110      A1 N2

 

[3 marks] 

d.

Examiners report

[N/A]
c.
[N/A]
d.

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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