A couple of months ago, it was a color-changing dress that blew out the neural circuits of the Internet. Though it may not have quite the mass appeal, this week it is a math problem that is making bushels of brains hurt.
It started with a posting on Facebook, by Kenneth Kong, a television host in Singapore.
From there, people around the world have been trying to figure out Cheryl’s birthday, or at least wondering why she couldn’t just save everyone a lot of trouble and be more direct with Albert and Bernard.
The wording of the problem is terrible, so here is a clearer version, which makes some of the assumptions more obvious but which does not change any of the underlying logic of the problem:
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 19June 17 — June 18July 14 — July 16August 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?
Originally, Mr. Kong said this was a problem inflicted on fifth-graders, leading to hand-wringing that Singapore children were way better at math than everyone else in the world and worries that Singapore children were being mentally abused with convoluted logic at a young age.
It turned out the problem actually came from a math olympiad test for math-savvy high school-age students.
How would you fare in a room full of adolescent math competitors in Singapore?
(Once you think you have figured it out — or if you don’t know where to start — here is the answer and an explanation.)
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