Wednesday, December 14, 2011

The Mind Reader

I am thinking of a number between 1 and 9. You can ask me two yes/no questions and I will answer them truthfully.

You can't ask me any open questions, I will only answer “Yes” or “No”! However, if for some reason I cannot answer it, I will tell you “I don't know”.¹

What two questions should you ask me to find the number I'm thinking of?

¹ Let's pretend I'm a genius and that the difficulty of your question does not stop me from answering it. Also, me not knowing the answer is not the same as making me deal with an invalid answer! So having me divide by zero will do you no good. That's just cheating out of a valid yes/no question. Encoding the numbers as yes/no/don't know? Same story!