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2.4.8 Normal distribution as a model (A-level only)

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Question 14

The random variable WWW represents the initial nutrient concentration, in mg/L, in a series of botanical samples. The probability distribution for WWW is given in the following table:

www1368
P(W=w)P(W=w)P(W=w)0.450.20.20.15
a.

Show that E(W)=3.45E(W) = 3.45E(W)=3.45.

[2]
b.

Find Var(W)Var(W)Var(W).

[3]
c.

The random variable SSS represents the soil porosity index of the sample's medium. The probability distribution for SSS is given in the following table, where kkk is a constant:

sss245kkk
P(S=s)P(S=s)P(S=s)0.250.250.250.25

Name the probability distribution of SSS.

[1]
d.

Given that E(S)=E(W)E(S) = E(W)E(S)=E(W), find the value of kkk.

[2]
e.

The growth of a seedling, GGG mm, is modelled by the normal distribution G∼N(μ,σ2)G \sim N(\mu, \sigma^2)G∼N(μ,σ2). Researchers Alice and Bob each select a nutrient concentration for μ\muμ and a porosity index for σ\sigmaσ by sampling from the distributions of WWW and SSS respectively. A sample is considered 'successful' if its growth exceeds 5 mm. The researcher whose parameters result in a higher probability of success, P(G>5)P(G > 5)P(G>5), wins.

Alice obtained w=6w = 6w=6 and s=2s = 2s=2. Bob obtained s=5s = 5s=5. Determine the probability that Bob wins.

[3]
f.

Find the largest probability of success, P(G>5)P(G > 5)P(G>5), achievable in this experiment.

[2]
g.

Find the probability of a researcher achieving this maximum probability of success.

[1]

2.4.8 Normal distribution as a model (A-level only) Questions

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