7.21. For a group of 100 people, compute a. the expected number of days of the year that are birthdays of exactly 3 people; b. the expected number of distinct birthdays.

The expected number of selection is: [tex]\mathbf{E(x) =365\times ^{100}C_3 \times \frac{1}{365}^3 \times \frac{364}{365}^{97}}[/tex]
The expected number of distinct birthdays is: [tex]\mathbf{E(x) = 365\times (1 - (\frac{364}{365})^{100})}[/tex]

The given parameters are:

[tex]\mathbf{n = 100}[/tex] --- sample size

[tex]\mathbf{r = 3}[/tex] --- selection of 3 people

(a) The expected number of birthdays

The probability of a having a birthday on a certain day is:

[tex]\mathbf{p = \frac{1}{365}}[/tex]

The probability of not having a birthday on the day is:

[tex]\mathbf{q = \frac{364}{365}}[/tex]

For a selection of 3 people in 100, the expected number of selection is:

[tex]\mathbf{E(x) =Days \times ^{100}C_3 \times p^3 \times q^{100 - 3}}[/tex]

So, we have:

[tex]\mathbf{E(x) = Days \times ^{100}C_3 \times \frac{1}{365}^3 \times \frac{364}{365}^{100 - 3}}[/tex]

Rewrite as:

[tex]\mathbf{E(x) =Days \times ^{100}C_3 \times \frac{1}{365}^3 \times \frac{364}{365}^{97}}[/tex]

There are 365 days in a year.

So, the expression becomes

[tex]\mathbf{E(x) =365\times ^{100}C_3 \times \frac{1}{365}^3 \times \frac{364}{365}^{97}}[/tex]

(b) Expected number of distinct birthdays

The probability that a person from the has his birthday on a certain day is:

[tex]\mathbf{p = 1 - (\frac{364}{365})^{100}}[/tex]

For 365 days, the expected number of distinct birthdays is:

[tex]\mathbf{E(x) = Days \times (1 - (\frac{364}{365})^{100})}[/tex]

There are 365 days in a year.

So, the expression becomes

[tex]\mathbf{E(x) = 365\times (1 - (\frac{364}{365})^{100})}[/tex]