Porisma

Worked solution

Combine normal observations and choose a minimum sample size

The mass of a Munchi pear is normally distributed with mean 120 g and standard deviation 10 g.

(i) Two pears are chosen at random. Find the probability that one has mass between 100 g and 126 g while the other has mass greater than 130 g.

(ii) Ten pears are chosen at random. Find the probability that exactly three have mass greater than 122 g each.

The mass of a Fuchi apple is normally distributed with mean 115 g and standard deviation 8 g.

(iii) Find the probability that the total mass of three randomly chosen Fuchi apples exceeds three times the mass of one randomly chosen Munchi pear.

(iv) For a random sample of nn Fuchi apples, find the least nn such that the probability that its sample mean differs from 115 g by more than 4 g is not more than 0.3.

[2010/NYJC/II/12]

CJC · Discussion · Question 4

Answer

(i) 0.2230.223; (ii) 0.1960.196; (iii) 0.3250.325; (iv) least n=5n=5.

(i) Allow both orders of the disjoint events

Write M∼N(120,102)M\sim N(120,10^2). The two possible orders are disjoint, so the probability is 2P(100<M<126)P(M>130)≈0.2232P(100<M<126)P(M>130)\approx0.223, to 3 s.f., using independent fruit selections.

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(ii) Count the qualifying pears

p=P(M>122)=0.4207403122…p=P(M>122)=0.4207403122\ldots. If XX counts qualifying pears among 10, then X∼B(10,p)X\sim B(10,p) and P(X=3)=(103)p3(1−p)7≈0.196P(X=3)=\binom{10}{3}p^3(1-p)^7\approx0.196, to 3 s.f.

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(iii) Compare a sum with three times one observation

Let independent Fj∼N(115,82)F_j\sim N(115,8^2) and D=F1+F2+F3−3MD=F_1+F_2+F_3-3M. Then E(D)=3(115)−3(120)=−15E(D)=3(115)-3(120)=-15 and Var⁡(D)=3(82)+32(102)=1092\operatorname{Var}(D)=3(8^2)+3^2(10^2)=1092. Thus D∼N(−15,1092)D\sim N(-15,1092) and P(F1+F2+F3>3M)=P(D>0)≈0.325P(F_1+F_2+F_3>3M)=P(D>0)\approx0.325, to 3 s.f.

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(iv) Method 1: solve the normal quantile inequality

Fˉ∼N(115,64/n)\bar F\sim N(115,64/n) exactly. The condition P(∣Fˉ−115∣>4)≤0.3P(|\bar F-115|>4)\le0.3 is equivalent to P(−4<Fˉ−115<4)≥0.7P(-4<\bar F-115<4)\ge0.7. Standardising gives P(−n/2<Z<n/2)≥0.7P(-\sqrt n/2<Z<\sqrt n/2)\ge0.7, so n/2≥Φ−1(0.85)≈1.03643338\sqrt n/2\ge\Phi^{-1}(0.85)\approx1.03643338. Therefore n≥4.2968…n\ge4.2968\ldots and the least integer is 5.

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(iv) Method 2: compare integer table values

The source enters normalcdf(111,119,115,√(8²/X)) as a function of the integer sample size XX. Its parameter screen has lower 111, upper 119, mean 115 and standard deviation 82/X\sqrt{8^2/X}. The printed comparison table is:

nnProbability
40.6827
50.7364
60.7793

The first acceptable integer is 5. The screenshot also lists the following surrounding values; this table method is for an integer unknown.

Source screen values, to 4 decimal places:

nnProbability
10.3829
20.5205
30.6135
40.6827
50.7364
60.7793
70.8141
80.8427
90.8664
100.8862
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Common mistakes

Var(3M)=9Var(M), while the variance of three independent apples is 3Var(F). A normal population gives an exact normal sample mean even when n is below 30.

Solution notes

No marks are printed for this question; none are invented.

PDF 6 starts by repeating the final Q4 answer “Min n is 5” before Q5 begins; retain that shared solution page. All printed and calculator-screen table values are represented in the working.

Read the hint