Famously, in a group of 23 randomly chosen people, the chance is slightly higher than 50 percent that two will share a birthday.

Here’s an interesting variant: If the group consists of an equal number of girls and boys, what’s the minimum size at which it’s probable that a girl and a boy share a birthday?

Surprisingly, the answer is only 32 (16 girls and 16 boys). If we want a girl and a boy to share the same birth month, we need only 6 children before this becomes probable.

(Tony Crilly and Shekhar Nandy, “The Birthday Problem for Boys and Girls,” Mathematical Gazette 71:455 [March 1987], 19-22. See Shared Birthdays.)