What is the probability that at least 2 people have the same birthday in a group of 35 people? Assume that there are 365 days in a year.

The given data is that at least 2 people have the same birthday in a group of 35 people.

There are 365 possibilities for each person's birthday. Since there are 35 people in the group, there are ${365}^{35}$possibilities for the birthday.

Thus, the number of elements in the sample space are $n\left(S\right)={365}^{35}$

let E be the event that at least two people have same birthday and $\overline{E}$be the event no two people have the same birthday.

First choose one person at random. There are 365 possibilities for his or her birthday. Next, choose the second person. If no two people have the same birthday then there are 364 possibilities for his or her birthday. Similarly, for 35th person, there are 331 possibilities for his birthday.

Thus,$n\left(E\right)=365·364·363·.....331$

The probability of event $\overline{E}$is given by

$$\begin{gathered} P\left(\overline{E}\right)=\frac{n\left(\overline{E}\right)}{n\left(S\right)} \\ =\frac{365·364·.....331}{{365}^{35}} \\ =0.1856 \end{gathered}$$

Use the formula $P\left(\overline{E}\right)=1-P\left(E\right)$to find the probability that at least 2 people have the same birthday in a group of 35 people.

$$\begin{gathered} P\left(\overline{E}\right)=1-P\left(E\right) \\ P\left(E\right)=1-P\left(\overline{E}\right) \\ P\left(E\right)=1-0.1856 \\ P\left(E\right)=0.8144 \end{gathered}$$