Logic Problem Goes Viral (updated)

Logic Problem Goes Viral (updated)

Perhaps you saw this logic problem, purported to have been given to fifth graders in Singapore, flying around social media yesterday:

Logic puzzle singapore

That’s right: a logic problem has gone viral.

It turns out that the problem was from a math olympiad test for high-school students, but perhaps the “are you smarter than a fifth grader from Singapore” framing helped propel this puzzle around the world.

The New York Times rephrases the puzzle like so:

Albert and Bernard just met Cheryl. “When’s your birthday?” Albert asked Cheryl.

Cheryl thought a second and said, “I’m not going to tell you, but I’ll give you some clues.” She wrote down a list of 10 dates:

May 15 — May 16May 19

June 17June 18

July 14 July 16

August 14August 15August 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: I don’t know when your birthday is, but I know Bernard doesn’t know, either.

Bernard: I didn’t know originally, but now I do.

Albert: Well, now I know, too!

When is Cheryl’s birthday?

So when is it?

(The NYT analysis is linked to from here. Don’t cheat.)

And, as I asked here, “Is there a lesson here for those concerned with outreach and public philosophy? Are there other such problems that could gain such traction so quickly in social media?”

UPDATE (4/16/15): Check out the helpful diagrams by Audrey Yap (UVic)  and video by Barteld Kooi (Groningen) at Richard Zach’s Logic Blog.

UPDATE 2 (4/16/15): From mathematician Timothy Gowers: An attempt to devise a transfinite version of the Cheryl birthday puzzle.

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