Singapore School Logic Puzzle
14 April 2015
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This puzzle has been getting a lot of attention recently, for example on the Guardian and BBC websites. It is quite similar to some of the Enigma puzzles published here.
Here it is slightly paraphrased to improve readability:
Albert and Bernard have just become friends with Cheryl, and they want to know when her birthday is.
Cheryl gives them a list of 10 possible dates, one of which is her birthday:
May 15, May 16, May 19,
June 17, June 18,
July 14, July 16,
August 14, August 15, August 17.
Cheryl then says she is going to tell the month of her birthday to Albert (but not to Bernard), and the day of her birthday to Bernard (but not to Albert). She does this.
The following conversation then took place:
Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard does not know either.
Bernard: I didn’t know when Cheryl’s birthday is, but now I do.
Albert: Then I also know when Cheryl’s birthday is.
So, assuming they are all telling the truth and are all perfect logicians and have no further information, when is Cheryl’s birthday.