250 teenagers gather for an exam. Peeking at the roll, a math-inclined among them note that if they are split into 10 groups of 25 per alphabetical order, then in each group, there are two with the same birthday, which would be quite remarkable at college.
But that comes to no surprise. Why?
This is not a duplicate of this question. For one thing, the math is a little more involved due to the notion of groups. For another… well why spoil it? Just a micro-hint:
The question was not originally tagged lateral thinking for a reason: the thinking required should be mainstream when statistics are involved.