1. The problem statement, all variables and given/known data A group of n students decided to find out on what day of the week each of them was born. Find the probability that all of them were born on different days of the week if a)n=2, b)n=4, c)n=7, d)n=8. 3. The attempt at a solution Let's start with n = 2. First, I started out by finding the complement of A, that they are all born on the same day. The total number of combinations is nr , where r = 7 for 7 days of the week. Then the probability is 7 choose 2, because there are 7 different possible days, and there are 2 students that were born on the same day. Thus, the probability of A' is 21/128 = .1641, and the probability of A is 1 - .1641 = .8359. This just doesn't seem right, because if you take n = 8, 8^7 = 2,097,152 but 7 choose 8 doesn't exist, so I'm thinking I'm misinterpreting the formulas or something.