When is Cheryl's birthday?' The tricky maths problem that has everyone stumped
It was meant to be a question to test teenagers at an elite high school in Singapore but it has baffled adults worldwide, triggering a string of attempts to solve it.
The question, about a girl named Cheryl asking two boys to guess her birthday, was first given as part of a test organised by the Singapore and Asian School Math Olympiads (SASMO).
However, it quickly went viral after a local broadcaster posted it on Facebook over the weekend.
In the question, Cheryl gives her new friends - Albert and Bernard - ten possible dates to choose from when they inquire about her date of birth. She then tells Albert the month and Bernard the day of her birthday.
Test-takers are then asked to use logic to deduce Cheryl's birthday using a short conversation between the two boys about the information given to them.
By Tuesday online users were engaged in serious debates over the best possible answer to the riddle on social media platforms like Reddit, Facebook and Twitter. "Updated my resume to add 'deduced Chery's birthday in five minutes," Phil Smith wrote on Twitter. Others weren't as clever. "I am stumped... when exactly is Chery's birthday?" Ian Gessey wrote online.
Bewildered by this brain teaser? Don't feel bad as most would fail to answer it. "An average student in the UK would panic if presented with this problem and would not even attempt it because pure logic-based questions are not part of the curriculum," says Karen Skuse, a former maths teacher at Langley school, an independent school in Norwich, and a maths veteran with 14 years teaching experience.
Some schools, however, would rise to the challenge. Carole Knight, a deputy head at Headington School said: "About a third of our girls, our top sets, would see this sort of Maths challenge routinely and relish it. They would enjoy rising to the challenge and we would use it as a great example of collaborative problem solving."
In a Facebook post late Monday, SOSMA provided the full question and a model answer, which can be found here. 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.
Karin McGourty, a primary school teacher, explains to the Telegraph's Raziye Akkoc how to work the puzzle out.
"I teach my students to use a table to lay out the information. It helps to organise their mind."
Readers here should create a table with two rows. A table of four columns with the months at the top and the dates Cheryl gives after (May 15, 16, 19; June 17, 18; July 14, July 16; August 14, 15 and 17)
Ms McGourty explains that Albert knows which month, so he knows if it is May or June. Bernard knows the date but if it was 18 or 19, they only appear once, and he would know the answer. But he doesn't.
"You can rule out some of the options. For Albert to have known the answer, he would have to have May and June as that is when 19 or 18 occur [respectively]."
This means it is not May or June.
"So Albert must have a month where numbers are repeated, leaving us with July and August," Ms McGourty adds. Bernard knows Albert has either of those two months and the numbers left include 15 through to 17 and 14 is repeated.
The number 14 is the only one in both months but Bernard is now sure of the birth date. This means Bernard knows it is July 16.
(China Daily 05/09/2015 page23)
