I found this puzzle while surfing around, it's supposedly very hard to solve and totally out of my league.
"No one can solve this. Not Ken Jennings. Not Marilyn vos Savant. Not Alan Turing. Not Ada Lovelace. Not Watson. Not even Richard Feynman. (Ok, maybe Feynman.)"
Whether you solve it and want to check your answer or you're just curious, here's the answer.