There are 33 students in a class. What is the probability that at least two of the 9 randomly selected people were born in the same month?
I know classical birthday problem ( 1-(365/365.)(364/365)...) but I couldn't apply this to this problem. Minimum 2 person and maximum 9 person and month. These are the differences.
The first thing to realise is that the 33 is irrelevant, ask yourself, how would the outcome change if I chose 9 people from 100? Or from 1000? The answer is, it wouldn't (with some assumptioms about the distribution)
So our new problem is simply: what is the probability that 9 people don't share more than one birthday month among them? hopefully you can see how to apply the method for finding the solution to the birthday problem now? (Hint, think of your year as being 12 days, and obviously your group is 9 people, how many ways can you distribute them with certain requirements?)