A maths problem that first appeared in a test for Singapore's elite high school students has baffled Internet users around the world after it went viral, prompting a rush of attempts to solve it.
The question, involving a girl asking two boys to guess her birthday after giving them scant clues, first appeared in an April 8 test organised by the Singapore and Asian School Math Olympiads (SOSMA).
It was meant for 15- and 16-year-old elite secondary school students, but swiftly went global after a local television news presenter posted it on his Facebook page on Saturday. Here is the question:
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!
By Monday Internet users around the world were posting meticulously detailed answers to the puzzle on social media networks such as Facebook and Reddit, only to prompt a slew of comments disputing their findings and methodology.
Others posted sardonic comments about “coy Cheryl”.
“Cheryl obviously didn't want Albert and Bernard at her party. Should have taken the hint when Cheryl decided to play this little game,” wrote David Leong on Facebook.
The “mind-boggling” problem also made the mainstream media, with leading websites including the Guardian newspaper and Buzzfeed publishing articles about the puzzle.
In a Facebook post late on Monday, SOSMA provided the full question and a model answer, which can be found at https://www.facebook.com/4sasmo/posts/983396811695295.
The post also clarified earlier reports that the question was posed to primary schoolchildren, saying SOSMA thought it important to specify the age of the students involved so “Singapore parents will not start to worry so much”.
The question was “actually from the secondary 3 and secondary 4 SASMO contests held on April 8, 2015”, it said, adding it was “meant to sift out the better students”.
Singapore is renowned worldwide for its national maths system, which has been emulated by schools in other developed countries and cities, including New York.
AFP