Cheryl’s Birthday Puzzle Part Two, Denise’s Revenge – Can You Solve It?

All those who have been dealing with weekday blues, here is an interesting puzzle which will cheer you up. Many puzzles of this kind were coined by a man named Joseph Yeo Boon Wooi, who is associated with the ‘National Institute of Education’, located in Singapore.

Quite recently, Yeo had framed a tricky riddle for a question paper of the ‘Singapore and Asian Schools Math Olympiad’. Soon, this question (famously known as ‘Cheryl’s Birthday Problem’) went online and became a topic of discussion amongst lovers of brain teasers. This question, which forces one to wear his thinking hat, involves three characters named Albert, Cheryl and Bernard.

Joseph Yeo is now back with a new mathematical problem, which is more like a sequel to the popular ‘Cheryl’s Birthday Problem’. An exception is an additional person named Denise, along with Albert, Bernard and Cheryl. Here is the description of this new problem and we wish you the best of luck to arrive at a solution.

Cheryl, Bernard and Albert have now got a new friend named Denise. The trio is curious to know the date of birth of their new friend. Denise gives them 20 different options and challenges them to guess the right answer. These are the dates given by Denise

17 Feb 2001, 16 Mar 2002, 13 Jan 2003, 19 Jan 2004

13 Mar 2001, 15 Apr 2002, 16 Feb 2003, 18 Feb 2004

13 Apr 2001, 14 May 2002, 14 Mar 2003, 19 May 2004

15 May 2001, 12 Jun 2002, 11 Apr 2003, 14 Jul 2004

17 Jun 2001, 16 Aug 2002, 16 Jul 2003, 18 Aug 2004

Later Denise revealed the month, the day and the year of her birth to Albert, Bernard and Cheryl respectively. The following conversation ensues:

Albert: I don’t know when Denise’s birthday is, but I know that Bernard does not know.

Bernard: I still don’t know when Denise’s birthday is, but I know that Cheryl still does not know.

Cheryl: I still don’t know when Denise’s birthday is, but I know that Albert still does not know.

Albert: Now I know when Denise’s birthday is.

Bernard: Now I know too.

Cheryl: Me too.

So, when is Denise’s birthday?

For your information: Albert has been told the month, Bernard knows the day and Cheryl is aware of the year. Neither of them have a clue of the other two details of Denise’s date of birth.

Trivia: An interesting fact about the ‘Cheryl’s Birthday Problem’ is that the answer to this puzzle is the same as Dr. Yeo’s date of birth. He has also hidden a fun little nugget – what he calls an “Easter egg” – in this one. It is something purely mathematical. What is it?