Sam lies on Mondays, Tuesdays and Wednesdays and tells the truth on other days. One day he says: 'Yesterday I lied.' Which days could it be?
Hint: Check the day after his lying run, and the first day of it.
Monday or Thursday.
On Thursday he truthfully says he lied on Wednesday. On Monday he lies, because he told the truth on Sunday. No other day works.
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