How to solve Singapore's viral logic problem
A school maths question posted on Facebook by a Singaporean TV presenter has stumped thousands worldwide.
The question asks students to guess the birthday of a girl called Cheryl using the minimal clues she gives to her friends, Albert and Bernard.
Author and mathematician Alex Bellos demonstrates how to solve the puzzle.
Here is the question:
Albert and Bernard just become friends with Cheryl, and they want to know when her birthday is.
Cheryl gives them a list of 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?