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Date May 2022 Marks available 8 Reference code 22M.2.SL.TZ1.5
Level Standard Level Paper Paper 2 Time zone Time zone 1
Command term Find Question number 5 Adapted from N/A

Question

The aircraft for a particular flight has 7272 seats. The airline’s records show that historically for this flight only 90%90% of the people who purchase a ticket arrive to board the flight. They assume this trend will continue and decide to sell extra tickets and hope that no more than 7272 passengers will arrive.

The number of passengers that arrive to board this flight is assumed to follow a binomial distribution with a probability of 0.90.9.

Each passenger pays $150$150 for a ticket. If too many passengers arrive, then the airline will give $300$300 in compensation to each passenger that cannot board.

The airline sells 7474 tickets for this flight. Find the probability that more than 7272 passengers arrive to board the flight.

[3]
a.

Write down the expected number of passengers who will arrive to board the flight if 7272 tickets are sold.

[2]
b.i.

Find the maximum number of tickets that could be sold if the expected number of passengers who arrive to board the flight must be less than or equal to 7272.

[2]
b.ii.

Find, to the nearest integer, the expected increase or decrease in the money made by the airline if they decide to sell 7474 tickets rather than 7272.

[8]
c.

Markscheme

(let TT be the number of passengers who arrive)

(P(T>72)=)  P(T73)(P(T>72)=)  P(T73)   OR   1-P(T72)1P(T72)         (A1)

T~B(74, 0.9)T~B(74, 0.9)   OR   n=74n=74         (M1)

=0.00379   (0.00379124)=0.00379   (0.00379124)        A1 


Note: Using the distribution B(74, 0.1)B(74, 0.1), to work with the 10%10% that do not arrive for the flight, here and throughout this question, is a valid approach.

 

[3 marks]

a.

72×0.972×0.9         (M1)

64.864.8        A1 

 

[2 marks]

b.i.

n×0.9=72n×0.9=72         (M1)

80        A1 

 

[2 marks]

b.ii.

METHOD 1

EITHER

when selling 7474 tickets

top row        A1A1

bottom row        A1A1


Note: Award A1A1 for each row correct. Award A1 for one correct entry and A1 for the remaining entries correct.


E(I)=11100×0.9962+10800×0.00338+10500×0.00041111099E(I)=11100×0.9962+10800×0.00338+10500×0.00041111099         (M1)A1


OR

income is 74×150=1110074×150=11100         (A1)

expected compensation is

0.003380...×300+0.0004110...×600  (=1.26070...)0.003380...×300+0.0004110...×600  (=1.26070...)         (M1)A1A1

expected income when selling 7474 tickets is 11100-1.26070111001.26070         (M1)

=11098.73  (=$11099)=11098.73  (=$11099)        A1 


THEN

income for 7272 tickets =72×150=10800=72×150=10800         (A1)

so expected gain 11099-10800=$2991109910800=$299        A1 

 

METHOD 2

for 7474 tickets sold, let CC be the compensation paid out

P(T=73)=0.00338014,  P(T=74)=0.000411098P(T=73)=0.00338014,  P(T=74)=0.000411098        A1A1

E(C)=0.003380×300+0.0004110×600  (=1.26070...)E(C)=0.003380×300+0.0004110×600  (=1.26070...)         (M1)A1A1

extra expected revenue =300-1.01404-0.246658  (300-1.26070)=3001.014040.246658  (3001.26070)         (A1)(M1)


Note: Award A1 for the 300300 and M1 for the subtraction.


=$299=$299   (to the nearest dollar)        A1

 

METHOD 3

let DD be the change in income when selling 7474 tickets.

         (A1)(A1)


Note: Award A1 for one error, however award A1A1 if there is no explicit mention that T=73T=73 would result in D=0D=0 and the other two are correct.


P(T73)=0.9962,  P(T=74)=0.000411098P(T73)=0.9962,  P(T=74)=0.000411098        A1A1

E(D)=300×0.9962+0×0.003380-300×0.0004110E(D)=300×0.9962+0×0.003380300×0.0004110         (M1)A1A1

=$299=$299        A1 

 

[8 marks]

c.

Examiners report

In part (a) Stronger candidates were able to recognize that they needed to use the binomial to find the probability. Some candidates confused binomialpdf and binomialcdf functions. Some did not understand that “more than 72” means “73 or 74” and how their GDC uses the lower boundary parameter.

In part (b) many candidates could find the expected number of passengers and the maximum number of tickets. This part was well attempted.

Part (c) was expected to challenge the strongest candidates and had little scaffolding. However, this may have been too much for this cohort and resulted in few marks being awarded. A variety of methods were used but few progressed beyond finding values of income minus compensation. Only a few candidates took probabilities into consideration.

a.
[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

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