You may solve the simple probability, *“Given a set of n randomly chosen people, what is the probability of two people having same birthday?”*

You may also solve, * “At least how many people need to be picked to make the probability 100% that at least two of them have same birthday?”*

Answer to the second question is 367, because there are 366 possible birthdays (including Feb-26).

The question now is, *“At least how many people need to be picked to make the probability 99.9% that at least two of them have same birthday?”*

Answer is 70.

And if the question is,*“At least how many people need to be picked to make the probability 50% that at least two of them have same birthday?”*

Answer is 23.