A 20-something patron walked up to me at the Reference Desk one evening and said,
You know that bulletin board in your teen area that always has different stuff on it? I don't know if you take suggestions from people, but here's a puzzle kids might like to try to figure out. I say "try" because I show this to lots of people and no one has solved it - even math teachers.
With that, he takes a piece of scrap paper, writes four 9s on it (as in, just 9999), and explains the puzzle.
The goal is to use these four nines, and any mathematical symbols, and have the result equal 100. You can use any combination of symbols - +, -, /, x, ( ), etc. - but the result must work out to be exactly 100.
Just then his ride came to get him, so he flipped the paper over, wrote the answer on the back, and said he hoped the kids would have fun with it.
So there I was - it was a slow night, I've got an "unsolvable" puzzle in front of me, and the answer is also at my finger tips. Such a temptation to cheat, but I gave it my best shot, trying all kinds of different ideas over the course of the night. No matter what I tried though, I just couldn't get it, so eventually I had to look at the answer.
I'm sure this puzzle (and the solution) is on the internet somewhere, but if you're interested, give it a try. If you give up, or want to check your answer, this link will launch the solution in a new window (this is exactly what the patron wrote on the back of the scrap paper).