Which Twin is Telling the Truth?
Date: 3/2/96 at 9:35:34 From: sarahd Subject: which twin is telling the truth? This is more of an English question, but it is in my math book: A man is walking to a party. There is a fork in the road. He doesn't know which way to go. At the fork in the road are identical twins. One always lies and One always tells the truth, but you don't know who tells what. If you can only ask one question (the same question to each twin), what is it? _________________________________________________? Why would you ask that: _________________________________________________?
Date: 3/4/96 at 21:39:51 From: Doctor Ken Subject: Re: which twin is telling the truth? Hello there! Actually, I'd say that this question is a perfectly good math question, in the part of math called Logic (some people do like to think of Logic as a field of Philosophy, though). Anyway, what's true about this question is that you actually only need to ask _one_ of the twins your question, you don't even need to ask both of them. You'll get enough information from one to make your decision. Let me give you a couple of hints, and then see whether you can take it from there. Since you don't know which twin you're talking to, it seems like you're not going to be able to tell whether your answerer is lying or telling the truth, and therefore whether the answer is true or false. It seems to be a lost cause. BUT WAIT! You actually _can_ solve this problem! Try to get the guy you ask to incorporate what the _other_ guy would say to your question. Then, if that works, try to figure out why (I mean, think about the problem even after you solve it). Good luck! [See also the Classic Problem "Liars and Truthtellers" in the Dr. Math FAQ: http://mathforum.org/dr.math/faq/faq.liar.html .] -Doctor Ken, The Math Forum
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