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paleonaps95 wrote:I think that (the sibling one) may be the only foolproof one, because the guard could say no for two as well as one.

have the number 1 or 2 in mind. Is the number that you have in mind larger than the number I have in mind?” If the guard answers “yes” then you know that they are thinking of number 3. If the guard says “I don’t know” then he is thinking of number 2. If the guard says “no” then he is thinking of number 1.

His number is 3: 3 > 2 and 3 > 1 so he answers yes
His number is 1: 1 < 2 and 1 = 1 (1 is not less than 1) so he answers no
His number is 2: 2 = 2 (2 is not less than 2) which would mean no, but 2 > 1 which would mean yes. Because it could be yes or no his response becomes I don't know.

Its foolproof, if that doesn't make sense, which it probably doesn't, someone else has explained this well already:
"If the guard has 2 in his mind, then he can’t answer “No” for certain, since if you’ve said that you have 1 or 2 in your head. If you have 2, then he could say no, but if you have 1 he’d say yes; not knowing which of these is in your head he’d need to say “I don’t know”.."

Sorry, I looked it up and saw a version where you had two in mind. Now I gets it.

How did this become the riddles thread? I'm good at competition math, not riddles

Math riddles is fun.

gh wrote:Don't remember what this was from, but it might have been something from NYCIML. Find the next term in the sequence:

By the way, the next in the sequence is Chambers St.