Tricky Math Problem Goes Viral

Can you figure out when Cheryl’s birthday is?  A puzzling logic problem for middle school “mathletes” in Singapore has gone viral online.  After a television host in Singapore posted the question to his Facebook page over the weekend, people around the world have taken a stab at it.

Here’s the question:  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 and August 17. Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.  Albert: I do not 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?  

Can you figure out the answer?

Hint: Albert is initially confident that Bernard doesn’’t know the answer, so the day can’t be a number that is only listed once.

The answer is below

(The correct answer is July 16. Go to Kong’s Facebook page for a detailed explanation.)