One person says: —Yesterday I lied. You know that this person always lies on Mondays, Tuesdays and Wednesdays, and always tells the truth on other days. What day of the week could it be today?
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The mentira of the monday
A quote about the calendar may seem innocent, but if someone always lies on certain days and tells the truth on others, the day of the week is caught between two possibilities.
Hints
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- The phrase speaks of yesterday, not today.
- If today says the truth, then yesterday tuvo that ser a day of mentira.
- If today miente, then yesterday no pudo ser a day of mentira.
Solution
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Answer: today could be Monday or Thursday. The person says: “Yesterday I lied.” If today he tells the truth, then the sentence must be true: yesterday he lied. That can only happen if today is Thursday, because yesterday was Wednesday, one of his lying days. If he lies today, then the sentence must be false: he did not lie yesterday. Among his lying days—Monday, Tuesday and Wednesday—that only happens on Monday, because yesterday was Sunday and on Sunday he tells the truth. Therefore, the only possible days are: - Monday;
- Thursday. On Tuesday or Wednesday I would be lying, but yesterday would also have been a day of lying, so the phrase “yesterday I lied” would be true. That cannot happen on a lying day. On Friday, Saturday or Sunday he would be telling the truth, but yesterday would not have been a day of lying, so the sentence would be false. It can't happen either.
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