Another beautiful puzzle this time – one from my nephew Karan Sharma, originally seen from Khan Academy, though available at other places as well. This is a truth-liar problem. Here it goes:
You are in a room with five people. One of them will always tell you the truth. The other four will toggle between telling you the truth, and lying to you. However they can either tell you the truth OR lie on the first question you ask them. After that they must always toggle between that and the opposite. For example. I see one has a mustache, and he is a toggler. I ask – “Do you have a mustache?” “Yes.” I ask again. “No.” I ask again. “Yes.” OR I ask – “Do you have a mustache?” “No.” I ask again. “Yes.” I ask again. “No.” The challenge of the riddle is to ask exactly TWO questions to anyone. You can ask both questions to the same person or ask questions to two different people, and at the end of your two questions you must find out who the truth-teller is.
Please send your answers either directly on the blog site as comments, or to me at firstname.lastname@example.org. If you like the puzzle, please share it with others. If you have interesting puzzles to share, please send them to me at my e-mail given above.