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Worked solution

Add a fixed box mass and retain both at-least-one methods

Pineapple masses in a large warehouse are normally distributed with mean 1.52 kg and standard deviation 0.070 kg. Six randomly chosen pineapples are packed in a box of mass 2.15 kg. Find the probability that the filled box has mass below 11.2 kg.

For 3 randomly chosen filled boxes, find the probability that at least 1 has mass below 11.2 kg.

[2005/II/23]

CJC · Discussion · Question 9

Answer

One filled box:0.342; at least one of three:0.715 (3 s.f.).

Model six independent pineapple masses

Let A∼N(1.52,0.0702)A\sim N(1.52,0.070^2) be a pineapple mass in kg. Under the source independent-sampling model, A1+⋯+A6∼N(1.52×6,0.0702×6)=N(9.12,0.0294)A_1+\cdots+A_6\sim N(1.52\times6,0.070^2\times6)=N(9.12,0.0294).

How did this part go?

Shift the threshold by the fixed box mass

P(A1+⋯+A6+2.15<11.2)=P(A1+⋯+A6<9.05)=0.341545≈0.342P(A_1+\cdots+A_6+2.15<11.2)=P(A_1+\cdots+A_6<9.05)=0.341545\approx0.342 to 3 significant figures. The fixed packaging adds no variance.

How did this part go?

Method 1: complement over three independent boxes

P(at least one below11.2kg)=1−P(none below11.2kg)=1−(1−0.341545)3≈0.715P(\text{at least one below11.2kg})=1-P(\text{none below11.2kg})=1-(1-0.341545)^3\approx0.715 to 3 significant figures. Distinct boxes are treated as independent with the same probability.

How did this part go?

Method 2: binomial count

Let X count the filled boxes below 11.2 kg. Then X∼B(3,0.341545)X\sim B(3,0.341545) and P(X≥1)=1−P(X=0)=0.714518≈0.715P(X\ge1)=1-P(X=0)=0.714518\approx0.715 to 3 significant figures. Both methods retain the source intermediate probability, rather than prematurely using 0.342.

How did this part go?

Common mistakes

Use the unrounded one-box probability when computing the three-box probability.

Solution notes

Both asks, full source sum distribution/threshold shift, both source alternative methods, all intermediate and rounded values retained. Independent fruit/box sampling assumptions explicit; no marks printed.

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