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Date May 2019 Marks available 3 Reference code 19M.2.SL.TZ2.S_9
Level Standard Level Paper Paper 2 Time zone Time zone 2
Command term Find Question number S_9 Adapted from N/A

Question

At Penna Airport the probability, P(A), that all passengers arrive on time for a flight is 0.70. The probability, P(D), that a flight departs on time is 0.85. The probability that all passengers arrive on time for a flight and it departs on time is 0.65.

The number of hours that pilots fly per week is normally distributed with a mean of 25 hours and a standard deviation σ. 90 % of pilots fly less than 28 hours in a week.

Show that event A and event D are not independent.

[2]
a.

Find P(AD).

[2]
b.i.

 Given that all passengers for a flight arrive on time, find the probability that the flight does not depart on time.

[3]
b.ii.

Find the value of σ.

[3]
c.

All flights have two pilots. Find the percentage of flights where both pilots flew more than 30 hours last week.

[4]
d.

Markscheme

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

METHOD 1

multiplication of P(A) and P(D)     (A1)

eg   0.70 × 0.85,  0.595

correct reasoning for their probabilities       R1

eg   0.5950.65,   0.70×0.85P(AD)

A and D are not independent      AG N0

 

METHOD 2

calculation of P(D|A)       (A1)

eg   1314,  0.928

correct reasoning for their probabilities       R1

eg   0.9280.85,   0.650.7P(D)

A and D are not independent      AG N0

[2 marks]

a.

correct working       (A1)

eg   P(A)P(AD) ,  0.7 − 0.65 , correct shading and/or value on Venn diagram

P(AD)=0.05       A1  N2

[2 marks]

 

b.i.

recognizing conditional probability (seen anywhere)       (M1)

eg   P(DA)P(A),  P(A|B)

correct working       (A1)

eg    0.050.7

0.071428

P(D|A)=114 , 0.0714     A1  N2

[3 marks]

b.ii.

finding standardized value for 28 hours (seen anywhere)       (A1)

eg   z=1.28155

correct working to find σ       (A1)

eg    1.28155=2825σ28251.28155

2.34091

σ=2.34     A1  N2

[3 marks]

c.

P(X>30)=0.0163429       (A1)

valid approach (seen anywhere)        (M1)

eg   [P(X>30)]2 ,  (0.01634)2 ,  B(2, 0.0163429) , 2.67E-4 , 2.66E-4

0.0267090

0.0267 %    A2  N3

[4 marks]

d.

Examiners report

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

Syllabus sections

Topic 4—Statistics and probability » SL 4.6—Combined, mutually exclusive, conditional, independence, prob diagrams
Show 306 related questions
Topic 4—Statistics and probability » SL 4.9—Normal distribution and calculations
Topic 4—Statistics and probability

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