I saw this on a t-shirt selling website. I had to get a hint before I could figure out the answer. It’s a tough one, and you may need to know more of math than the average high school graduate to answer it. I’m not talking down to people; I would have had NO idea how to answer this after high school. In fact, a 300 level college course is the first time I heard about the math behind this one.
But do not be disheartened! A little research at Wikipedia or somewhere else could catch you up on the required knowledge.
So here it is.

How many people know hexadecimal if only you and dead people know hexadecimal?

Best of luck. Let me know your answers. I’ll try not to post the answer too quickly in concern of ruining the opportunity for someone else. If you leave a name or email or some way to get back to you, I could verify the answer that way. Up to you.
Pleasant thinking to all.