Elementary Statistics (12th Edition)

a) The selections of birthdays are independent, hence we can us the multiplication rule: P(all bithdays are different)=$\frac{365}{365}\cdot\frac{364}{365}\cdot\frac{363}{365}\cdot...\frac{341}{365}=0.43.$ b)Here we can use the complement rule: P(at least 2 birthdays are the same=1-P(all bithdays are different)=1-0.43=0.57.