Q: Here is the Math/logic brain teaser that took the Internet by storm in mid-April 2015:
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: May 15 May 16 May 19 June 17 June 18 July 14 July 16 August 14 August 15 August 17
Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively. Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard does not know too.
Bernard: At first I don’t know when Cheryl’s birthday is, but I know now.
Albert: Then I also know when Cheryl’s birthday is.
So when is Cheryl’s birthday?
A: July 16th.
Can’t be a unique day. Therefore it cannot be May or June.
July 14 July 16
August 14 August 15 August 17
Has to be unique
August 15 August 17
If Bernard know and Albert now knows that it has to be July 16th. If it was August then Albert would not know.
(Thanks to Adam Schwartz for this solution!)