The puzzle: Pick a random day from a random year. If it turns out to be the 13th of some month, what day of the week is it most likely to be?
A solution: Although it seems like no one day should be favoured over another, it turns out there is a (very slight) preference in favour of ... Friday!
This fact was first observed by B.H. Brown, a professor of mathematics at Dartmouth in the 1930's. To see a proof, have a look at this powerpoint.
I have no idea how to choose randomly between two options, so: both Sean and Gabe are winners.