Talk:Puzzles/Logic puzzles/Lying about your Age

Lying about your Age

Carrie said to Annie: "I'm ten years older than you."

''I realized that when one of them spoke to somebody older than themselves, everything they said was true, but when speaking to somebody younger, everything they said was false. ''

Two possibilities: Carrie is older (speaking to younger) OR Carrie is younger (speaking to older).

If Carrie is older and speaking to someone younger, then everything she said is false. Therefore, Carrrie cannot be older. If Carrie is younger and speaking to someone older, then everything she said is true. Therefore, she is older. But we originally assumed that she was younger. Reductio ad absurdum. Lord Emsworth 00:54, 11 Oct 2003 (UTC)


 * The second argument is nice: If Carrie would be younger then she would be telling the truth (i.e. implying that she is 10 years older which leads to a contradiction). This is the kind of reasoning you'll need! The first argument is not valid, I think: If Carrie is older, then the sentence "I'm ten years older than you." must be false. This does NOT mean that Carrie is younger, it could be that Carrie is 5 years or 20 years older! (I'm not sure if I said this in the problem, but you can assume that all persons have a different age). Thomas


 * Ah... The sentence "when speaking to somebody younger, everything they said was false" might have been misleading.


 * I see. I don't know though how to formulate it in a better way. If you have an idea, please feel free to edit it Thomas


 * Of course, it is fine now that it has been clarified. --Lord Emsworth 00:49, 12 Oct 2003 (UTC)

I think it needs to be clarified that ONLY one person lies, and ONLY when they are talking to someone younger. --Hitsthings (talk) 06:25, 9 April 2008 (UTC)