If I think of a number between 1 and 1,000, guessing it in 10 yes-no questions is easy... so that's not the puzzle!

Guessing it in 10 yes-no questions, that must be all asked in advance, is also relatively easy... so that's not the puzzle either!

How many questions would you need to guess my number, if you had to ask all questions beforehand, and I also could lie once?

15 questions required:

1-10 as stated by Charlie

11-14 as stated by e.g.

15: Did you lie in questions 11-14?

Question 15 is needed because 14 truthful answers are needed and you cannot guarantee the that one of the first 14 is not a lie.

Posted by Leming on 2005-04-14 21:03:21