pen rainbow

Thursday, April 16, 2015

Thursday Mini-Post: Cheryl's Birthday

This logic problem was originally posted by Kenneth Kong, a television host from Singpore.  It originated from a high school math olympiad.  

I solved it in 5 minutes by writing down the possibilities, then using a process of elimination to knock out the non-possibilities.  Pay attention to Albert's statement, "I don't know when your birthday is, but I know that Bernard does not know it either."  That tells you a lot.


Deductive Reasoning


Cheryl's Birthday

Albert and Bernard just met Cheryl.  

"When is your birthday?" Albert asked Cheryl.

Cheryl thought for a second and said, 

"I'm not going to tell you, but I'll give you some clues."  

She wrote down a list of ten dates:


  • May 15, May 16, May 19
  • June 17, June 18
  • July 14, July 16
  • August 14, August 15, August 17

"My birthday is one of these," she said.

Then, Cheryl whispered in Albert's ear the month—and only the month—of her birthday.  

To Bernard, she whispered the day, and only the day.

"Can you figure it out now?" she asked Albert.

Albert pondered, "I don't know when your birthday is, but I know that Bernard does not know it, either.

Bernard answered, "I did not know originally, but now I do."

Albert exclaimed, "Well, now I know, too!"


When is Cheryl's birthday?

Click HERE for the answer.