Consider a variation of the birthday problem: “What is the
probability that, in a group of n people, at least three have the
same birthday.†As with the original birthday problem, for large n
it is not computationally feasible to solve this variation by
setting up and scanning the sample space, nor by using counting
principles. However, this variation can easily be solved using
complement probability. Solve this variation of the birthday
problem using complement probability. A possible answer is given
here:
https://www.chegg.com/homework-help/questions-and-answers/-consider-variation-birthday-problem-probability-group-n-people-least-three-birthday-origi-q25476545?trackid=5a88fe81&strackid=62b06866&ii=3
But i dont understand the answer especially why the part where
exactly 2 people sharing the birthday has the denominator it has.
Thanks
