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Date November 2018 Marks available 7 Reference code 18N.1.SL.TZ0.S_9
Level Standard Level Paper Paper 1 Time zone Time zone 0
Command term Find Question number S_9 Adapted from N/A

Question

A bag contains nn marbles, two of which are blue. Hayley plays a game in which she randomly draws marbles out of the bag, one after another, without replacement. The game ends when Hayley draws a blue marble.

 Let nn = 5. Find the probability that the game will end on her

Find the probability, in terms of nn, that the game will end on her first draw.

[1]
a.i.

Find the probability, in terms of nn, that the game will end on her second draw.

[3]
a.ii.

third draw.

[2]
b.i.

fourth draw.

[2]
b.ii.

Hayley plays the game when nn = 5. She pays $20 to play and can earn money back depending on the number of draws it takes to obtain a blue marble. She earns no money back if she obtains a blue marble on her first draw. Let M be the amount of money that she earns back playing the game. This information is shown in the following table.

Find the value of kk so that this is a fair game.

[7]
c.

Markscheme

2n2n     A1 N1

 

[1 mark]

a.i.

correct probability for one of the draws      A1

eg   P(not blue first) = n2nn2n,   blue second = 2n12n1

valid approach      (M1)

eg   recognizing loss on first in order to win on second, P(B' then B),  P(B') × P(B | B'),  tree diagram

correct expression in terms of nn       A1 N3

eg   n2n×2n1n2n×2n12n4n2n2n4n2n,  2(n2)n(n1)2(n2)n(n1)

 

[3 marks]

a.ii.

correct working      (A1)

eg   35×24×2335×24×23

1260(=15)1260(=15)     A1  N2

 

[2 marks]

b.i.

correct working      (A1)

eg  35×24×13×2235×24×13×22

660(=110)660(=110)    A1  N2

 

[2 marks]

b.ii.

correct probabilities (seen anywhere)      (A1)(A1)

eg   P(1)=25P(1)=25,  P(2)=620P(2)=620  (may be seen on tree diagram)

valid approach to find E (M) or expected winnings using their probabilities      (M1)

eg   P(1)×(0)+P(2)×(20)+P(3)×(8k)+P(4)×(12k)P(1)×(0)+P(2)×(20)+P(3)×(8k)+P(4)×(12k),

P(1)×(20)+P(2)×(0)+P(3)×(8k20)+P(4)×(12k20)P(1)×(20)+P(2)×(0)+P(3)×(8k20)+P(4)×(12k20)

correct working to find E (M) or expected winnings      (A1)

eg   25(0)+310(20)+15(8k)+110(12k)25(0)+310(20)+15(8k)+110(12k),

25(20)+310(0)+15(8k20)+110(12k20)25(20)+310(0)+15(8k20)+110(12k20)

correct equation for fair game      A1

eg   310(20)+15(8k)+110(12k)=2025(20)+15(8k20)+110(12k20)=0

correct working to combine terms in k       (A1)

eg   8+145k42=0,  6+145k=20,  145k=14

k = 5    A1 N0

Note: Do not award the final A1 if the candidate’s FT probabilities do not sum to 1.

 

[7 marks]

c.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
b.i.
[N/A]
b.ii.
[N/A]
c.

Syllabus sections

Topic 4—Statistics and probability » SL 4.5—Probability concepts, expected numbers
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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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