Archive for November, 2009

A logic puzzle

Posted in General on November 27, 2009 by jkiparsky

A conversation over coffee and pie yesterday led me back, once again, to the logic puzzles of Raymond Smullyan, probably my favorite logician ever. This one comes out of “The Lady or the Tiger”, one of several collections of puzzles he put together, but he calls it an old one – you may have come across it.
He claims this puzzle is great for starting arguments, in that there are a number of wrong answers that people will defend vigorously. I’ll put the puzzle here, so you can have a nice argument with someone. If there’s any call for the answer, I’ll supply it…

A salesman buys a widget in the morning for $7, and sells it before lunch for $8. After lunch, he buys it back for $9, and before the end of the day he sells it again for $10. How much profit did he make on this widget?