4 points Parts $\boldsymbol{A}$ and $\boldsymbol{B}$ are independent. The requested probabilities will be given to $10^{-3}$ near. To help detect certain allergies, a blood test can be performed whose result is either positive or negative. In a population, this test gives the following results:
- If an individual is allergic, the test is positive in $97\%$ of cases;
- If an individual is not allergic, the test is negative in $95{,}7\%$ of cases.
Furthermore, $20\%$ of individuals in the concerned population have a positive test. We randomly choose an individual from the population, and we denote:
- $A$ the event ``the individual is allergic'';
- $T$ the event ``the individual has a positive test''.
We denote by $\bar{A}$ and $\bar{T}$ the complementary events of $A$ and $T$. We also call $x$ the probability of event $A$: $x = p(A)$.
Part A - Reproduce and complete the tree describing the situation, indicating on each branch the corresponding probability.
- a. Prove the equality: $p(T) = 0{,}927x + 0{,}043$. b. Deduce the probability that the chosen individual is allergic.
- Justify by a calculation the following statement: ``If the test of an individual chosen at random is positive, there is more than $80\%$ chance that this individual is allergic''.
Part BA survey on allergies is conducted in a city by interviewing 150 randomly chosen residents, and we assume that this choice amounts to successive independent draws with replacement. We know that the probability that a randomly chosen resident in this city is allergic is equal to $0{,}08$. We denote by $X$ the random variable that associates to a sample of 150 randomly chosen residents the number of allergic people in this sample.
- What is the probability distribution followed by the random variable $X$? Specify its parameters.
- Determine the probability that exactly 20 people among the 150 interviewed are allergic.
- Determine the probability that at least $10\%$ of the people among the 150 interviewed are allergic.