Answer
"Are you able to tell me that the ruins are down the left path?"
Work Step by Step
Villager:
either L=liar or T=truth-teller.
answer either Yes or No to one question.
One branch leads to the ruins you want to visit; the other leads deep into the jungle.
One villager (either L or T) stands at the fork in the road. What one question can you ask to determine which branch to take?
An example of a question that doesn't work:
"Does the left path lead to the ruins?" If it does:
L = No
T = Yes
Clearly, we want a question that is not directly tied to the yes/no oppositional structure.
Are you able to tell me that the ruins are down the left path?
If they are:
L: Yes - they can never tell the truth, and the ruins are down the left path, so they are not able to tell them
T: Yes
If they aren't:
L: No - they are able to say the ruins are down the left path, because they are not; however, they must lie about what they would have answered.
T: No
