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