Exercise 1 (5 points)
Part A
An oyster farmer raises two species of oysters: ``the flat'' and ``the Japanese''. Each year, flat oysters represent $15 \%$ of his production. Oysters are said to be of size $\mathrm { n } ^ { \circ } 3$ when their mass is between 66 g and 85 g. Only $10 \%$ of flat oysters are of size $\mathrm { n } ^ { \circ } 3$, whereas $80 \%$ of Japanese oysters are.
- The health service randomly selects an oyster from the oyster farmer's production. We assume that all oysters have an equal chance of being selected. We consider the following events:
- J: ``the selected oyster is a Japanese oyster'',
- C: ``the selected oyster is of size $\mathrm { n } ^ { \circ } 3$''.
a. Construct a complete weighted tree representing the situation. b. Calculate the probability that the selected oyster is a flat oyster of size $\mathrm { n } ^ { \mathrm { o } } 3$. c. Justify that the probability of obtaining an oyster of size $\mathrm { n } ^ { \circ } 3$ is 0.695. d. The health service selected an oyster of size $\mathrm { n } ^ { \circ } 3$. What is the probability that it is a flat oyster?
- The mass of an oyster can be modeled by a random variable $X$ following the normal distribution with mean $\mu = 90$ and standard deviation $\sigma = 2$. a. Give the probability that the oyster selected from the oyster farmer's production has a mass between 87 g and 89 g. b. Give $\mathrm { P } ( \mathrm { X } \geqslant 91 )$.
Part B
This oyster farmer claims that $60 \%$ of his oysters have a mass greater than 91 g.
A restaurant owner would like to buy a large quantity of oysters but would first like to verify the oyster farmer's claim.
The restaurant owner purchases 10 dozen oysters from this oyster farmer, which we will consider as a sample of 120 oysters drawn at random. His production is large enough that we can treat it as sampling with replacement. He observes that 65 of these oysters have a mass greater than 91 g.
- Let F be the random variable that associates to any sample of 120 oysters the frequency of those with a mass greater than 91 g. After verifying the conditions of application, give an asymptotic confidence interval at the $95 \%$ level for the random variable F.
- What can the restaurant owner think of the oyster farmer's claim?