If I think of a number between 1 and 1,000, guessing it in 10 yesno questions is easy... so that's not the puzzle!
Guessing it in 10 yesno 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:
110 as stated by Charlie
1114 as stated by e.g.
15: Did you lie in questions 1114?
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 20050414 21:03:21 