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Date May Specimen paper Marks available 6 Reference code SPM.1.AHL.TZ0.10
Level Additional Higher Level Paper Paper 1 Time zone Time zone 0
Command term Find Question number 10 Adapted from N/A

Question

In a coffee shop, the time it takes to serve a customer can be modelled by a normal distribution with a mean of 1.5 minutes and a standard deviation of 0.4 minutes.

Two customers enter the shop together. They are served one at a time.

Find the probability that the total time taken to serve both customers will be less than 4 minutes.

Clearly state any assumptions you have made.

Markscheme

let TT be the time to serve both customers and TiTi the time to serve the iith customer

assuming independence of T1T1 and T2T2      R1

TT is normally distributed and T=T1+T2T=T1+T2       (M1)

E(T)=1.5+1.5=3E(T)=1.5+1.5=3     A1

Var(T)=0.42+0.42=0.32Var(T)=0.42+0.42=0.32      M1A1

P(T<4)=0.961P(T<4)=0.961       A1

[6 marks]

Examiners report

[N/A]

Syllabus sections

Topic 4—Statistics and probability » AHL 4.15—Central limit theorem
Topic 4—Statistics and probability

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