Exercise 1: ProbabilityA company manufactures components for the automotive industry. These components are designed on three assembly lines numbered 1 to 3.
- Half of the components are designed on line $\mathrm{n}^{\circ}1$;
- $30\%$ of the components are designed on line $\mathrm{n}^{\circ}2$;
- the remaining components are designed on line $\mathrm{n}^{\circ}3$.
At the end of the manufacturing process, it appears that $1\%$ of the parts from line $\mathrm{n}^{\circ}1$ have a defect, as do $0.5\%$ of the parts from line $\mathrm{n}^{\circ}2$ and $4\%$ of the parts from line $\mathrm{n}^{\circ}3$. One of these components is randomly selected. We denote:
- $C_1$ the event ``the component comes from line $\mathrm{n}^{\circ}1$'';
- $C_2$ the event ``the component comes from line $\mathrm{n}^{\circ}2$'';
- $C_3$ the event ``the component comes from line $\mathrm{n}^{\circ}3$'';
- $D$ the event ``the component is defective'' and $\bar{D}$ its complementary event.
Throughout the exercise, probability calculations will be given as exact decimal values or rounded to $10^{-4}$ if necessary.
PART A - Represent this situation with a probability tree.
- Calculate the probability that the selected component comes from line $\mathrm{n}^{\circ}3$ and is defective.
- Show that the probability of event $D$ is $P(D) = 0.0145$.
- Calculate the probability that a defective component comes from line $\mathrm{n}^{\circ}3$.
PART BThe company decides to package the produced components by forming batches of $n$ units. We denote $X$ the random variable which, to each batch of $n$ units, associates the number of defective components in this batch. Given the company's production and packaging methods, we can consider that $X$ follows the binomial distribution with parameters $n$ and $p = 0.0145$.
- In this question, the batches contain 20 units. We set $n = 20$. a. Calculate the probability that a batch contains exactly three defective components. b. Calculate the probability that a batch contains no defective components. Deduce the probability that a batch contains at least one defective component.
- The company director wishes the probability of having no defective components in a batch of $n$ components to be greater than 0.85. He proposes to form batches of at most 11 components. Is he correct? Justify your answer.
PART CThe manufacturing costs of the components of this company are 15 euros if they come from assembly line $\mathrm{n}^{\circ}1$, 12 euros if they come from assembly line $\mathrm{n}^{\circ}2$ and 9 euros if they come from assembly line $\mathrm{n}^{\circ}3$. Calculate the average manufacturing cost of a component for this company.