1. Let $n ≥ 2$ be the number of people in a room. Each of these people have a uniformly random and independent birthday. The year has $365$ days (ignore leap years).
Define the event $A =$ “at least $2$ people have their birthday on December $14$”.
What is Pr(A)?
Answer: $1-(\frac{364}{365})^n -n \cdot \frac{1}{365} \cdot (\frac{364}{365})^{n-1}$
Can someone break down the answer and tell me what each part counts?
$Pr($at least 2$) = 1 - Pr($at least 1$) - Pr($ no birthdays on Dec. 14$)$
$Pr($at least 1 birthday on December 14$)$:
I think that $(\frac{364}{365})^n$ represents that there are $n$ people who have a choice of $364$ out of $365$ birthdays (excluding December 14). Since there are $n$ people and they all have a choice of $364$, then: $(\frac{364}{365})^n$
This is where I get confused:
$Pr($no birthdays on Dec. 14$)$
How does $n \cdot \frac{1}{365} \cdot (\frac{364}{365})^{n-1}$ count the people who do do not have a birthday on December 14th?
2. Let $X = \{1,2,..., n\}.$ We choose a uniformly random subset $Y$ of $X$ having size $17$.
Define the event:$A = 4 \in Y$ or $7 \in Y$
What is Pr(A)?
Answer: $\frac{2 \cdot \binom{99}{16} - \binom{98}{15}}{\binom{100}{17}}$
Can someone break this down as well? Where is the numerator coming from?
Thank you.