Internet in knots over 'Cheryl's birthday' logic problem

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By Katia Hetter

(CNN) -- Are you smarter than a really smart Singapore high school student?

See if you can figure out Cheryl's birthday, the Singapore logic problem that's got the Internet twisted into knots. Some are even saying it's the math equivalent of the "What color is the dress" debate.

The puzzling problem went viral after Singapore television host Kenneth Kong posted it to Facebook.

Cherl's birthday challenge was meant to test the better high-school students competing in the Singapore and Asian Schools Math Olympiad held on April 8.

Here it is: Cheryl has a birthday, and she doesn't want to share it right away. (Otherwise, we have no challenge to solve.) So she gives her friends Albert and Bernard a list of 10 possible birthday dates in May, June, July and August.

A brief conversation between Albert and Bernard -- who we assume are not lying for the sake of this problem -- gives the reader enough clues to eliminate nine of the 10 dates and discover Cheryl's birthday.

That's assuming you want to celebrate with Cheryl after she's put you through all that trouble.

The New York Times, the Washington Post have posted their solutions to the problem, along with explanations.

Of course, the Internet has offered other solutions.


The problem reads:

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?



There are only two dates with unique numbers: May 19 or June 18. If Bernard was given a date of either 18 or 19, he'd know right away. The fact that Albert says Bernard doesn't know means the month he was given wasn't May or June. Bernard has now figured this much out, and if he had been told 14 he wouldn't know the answer as there are still two 14s left (July and August) leaving July 16, August 15 and August 17. Bernard must've been given the number 16, because for Albert to say he knows too would rule out August, which still has two remaining dates. That leaves the answer: July 16.



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