Buffy thinks of a number, 1, 2 or 3. Willow is allowed to ask Buffy one yes-or-no question, to which she will answer “yes”, “no”, or, if she doesn’t know the answer, “I don’t know”.
What question should Willow ask Buffy to uncover his number?
Answer
One solution is for Buffy to ask Willow:
“I’m thinking of 1 or 2. Is your number higher than mine?”
If Willow has 3, then she will answer “yes”. If she has 1, he must answer “no”. And, if she has 2, she can’t know if his number is higher than Buffy’s, so she must answer “I don’t know”
Another solution is for Buffy to say:
“Say your number is x. Is the result of 1/(x-2) greater than 0?”
Source: puzzle.dse.nl.