Lost in the woods, you finally happen upon a rickety old bridge across a deep ravine. The ravine is too steep to go down and climb back up. You need to know if the bridge is safe. As 'luck' would have it, you recognize that on the other side of the bridge is that dreadful town, "Knight-and-Liarville". Everyone who lives there is a knight (who always tells the truth) or a liar (who always lies). You are tired and you've been lost in Knight-and-Liarville before. You see three men on the other side of the bridge.

You shout out: "Are you a knight?"

The first man says something, but you can't hear what he said.

The second man shouts, "He said he was a knight."

The third man shouts, "No, he didn't. He said he was a liar."

Which man do you ask to find out whether or not the bridge is safe?

The first man must have answered "Yes" to the question "Are you a Knight ?". Therefore the second man was telling the truth, and the third man was lying. So it doesn't matter whether you ask the second or the third man whether the bridge is safe. You will know it is safe if the second man says it is, and as well if the third man says it isn't. (This is assuming they have knowledge of whether the bridge is safe. This is generally assumed in puzzles like this. If we don't make that assumption, then if the third man didn't know whether the bridge is safe, and he can only lie when he speaks, he would either remain silent for fear of accidentally telling the truth, or he would give an evasive answer, such as "I know whether it is safe or not, and I will tell you the answer in 5 minutes". So even in that case the second man would say "I don't know whether the bridge is safe", and the third man would either remain silent or give an obviously evasive answer. So again I guess it doesn't matter which one you ask).

 

 

 

 

 

 

 

 

 

Edited on March 26, 2004, 10:04 am

Posted by Penny on 2004-03-26 09:48:08