Question about random variables I'm new in a probability course and I stuck with the following questions. The problems stipulate: In a store there are 10 printers such that 4 are defective, 6 are good and one company buy 5 of them. First question: What is the probability for choice 5 that are good, and I realize that this is a random variable whit hypergeometric distribution and the answer is 0.02381. Second question: If the company decides repair the defective printers at cost of 50 dollars each one, find the expectation and variance of the cost of such repairs.

**Basic approach.** Use the same hypergeometric distribution you used for the first question to determine

$$ p_k \stackrel{\text{def}}{=} P(\text{out of the $5$ chosen printers, exactly $k$ are bad}) $$

Note then that also $p_k = P(\text{repairs cost $50k$ dollars})$. So you can easily determine expected total cost $C$ as

$$ E(C) = \sum_{k=0}^4 50k p_k $$

Find $E(C^2)$ in a similar way, and then

$$ \sigma^2_C = E(C^2)-[E(C)]^2 $$