The liar and the truth-teller
bin Hassad, an ancient Arabic mathematician had to get to the Great Library at Alexandria in the shortest possible way to deliver a very important discovery in geometry. At one stage in his travels he came to a fork in the road. He knew that one of the roads would lead him hopelessly astray while the other would advance him on his journey.
Sitting on the side of the road at the fork was an unkempt man in tattered rags. bin Hassad had been forewarned about this. This man was either a total liar (said to be a man called Basdured) or another man who was known to be totally honest (called The Truth-Teller).
bin Hassad did not know which man he faced. He knew that both men, though not omniscient, were intelligent and knowledgeable about the geography. He also knew that he could only ask the man one question about the fork which could only be answered by a simple "yes" or a "no". He had come armed with a question to discover the way forward, avoiding the way that led to nowhere.
What did he ask?