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Monday, July 28, 2008

Troubled Traveler

A traveler, on his way to Eindhoven, reaches a road junction, where he can turn left or right. He knows that only one of the two roads leads to Eindhoven, but unfortunately, he does not know which one. Fortunately, he sees two twin-brothers standing at the road junction, and he decides to ask them for directions.

The traveler knows that one of the two brothers always tells the truth and the other one always lies. Unfortunately, he does not know which one always tells the truth and which one always lies.

The Question: How can the traveler find out the way to Eindhoven by asking just one question to one of the two brothers?


Answer: The question that the traveler should ask is: "Does the left road lead to Eindhoven according to your brother?" If the answer is "Yes", the traveler should turn right, and if the answer is "No", the traveler should turn left.

Explanation: There are four possible cases:

1. The traveler asks the question to the truth-telling brother, and the left road leads to Eindhoven. The truth-telling brother knows that his lying brother would say that the left road does not lead to Eindhoven, and so he answers "No".
2. The traveler asks the question to the truth-telling brother, and the right road leads to Eindhoven. The truth-telling brother knows that his lying brother would say that the left road leads to Eindhoven, and so he answers "Yes".
3. The traveler asks the question to the lying brother, and the left road leads to Eindhoven. The lying brother knows that his truth-telling brother would say that the left road leads to Eindhoven, and so he lies "No".
4. The traveler asks the question to the lying brother, and the right road leads to Eindhoven. The lying brother knows that his truth-telling brother would say that the left road does not lead to Eindhoven, and so he lies "Yes".


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