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 Fuchi apples, find the least 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) ; (ii) ; (iii) ; (iv) least .
(i) Allow both orders of the disjoint events
Write . The two possible orders are disjoint, so the probability is , to 3 s.f., using independent fruit selections.
(ii) Count the qualifying pears
. If counts qualifying pears among 10, then and , to 3 s.f.
(iii) Compare a sum with three times one observation
Let independent and . Then and . Thus and , to 3 s.f.
(iv) Method 1: solve the normal quantile inequality
exactly. The condition is equivalent to . Standardising gives , so . Therefore and the least integer is 5.
(iv) Method 2: compare integer table values
The source enters normalcdf(111,119,115,√(8²/X)) as a function of the integer sample size . Its parameter screen has lower 111, upper 119, mean 115 and standard deviation . The printed comparison table is:
| Probability | |
|---|---|
| 4 | 0.6827 |
| 5 | 0.7364 |
| 6 | 0.7793 |
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:
| Probability | |
|---|---|
| 1 | 0.3829 |
| 2 | 0.5205 |
| 3 | 0.6135 |
| 4 | 0.6827 |
| 5 | 0.7364 |
| 6 | 0.7793 |
| 7 | 0.8141 |
| 8 | 0.8427 |
| 9 | 0.8664 |
| 10 | 0.8862 |
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.