What is the probability that in a class of 30 people everyone has a different birthday.
Would the answer be $1-\frac{364}{365} \cdot \frac{363}{365}\cdot\frac{362}{365}\cdots$ as it is written in my book or would it be $\frac{365}{365}\cdot\frac{364}{365}\cdot\frac{363}{365}\cdots$ I dont understand where the $1 -{}$ comes from