Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is. Cheryl
gives them a list of 10 possible dates:
Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.
She secretly tells Albert the month in which her birthday falls (i.e. May, June, July, or August) and she
secretly tells Bernard the day of her birthday (14, 15, 16, 17, 18, or 19).
Albert and Bernard then make the following statements:
Albert: "I don't know when Cheryl's birthday is, but I know that Bernard does not know either."
Bernard: "At first I didn't know when Cheryl's birthday was, but now I know."
Albert: "Then I also know when Cheryl's birthday is."
So when is Cheryl's birthday?
(This clever logic puzzle was one of the questions on a Singapore and Asian Schools Math Olympiad that
went viral in April 2015.)
Do you have a suggestion
for this puzzle (e.g. something that should
be mentioned/clarified in the question or solution, bug, typo, etc.)?