A few days ago, Singaporean television host Kenneth Kong posted the below maths question on his Facebook page, and it went viral. The high school maths question was aimed at 15- to 17-year-olds, but somehow it managed to stump half the Internet. It even appeared on the New York Times science page, and prompted a blog from British author and numbers expert Alex Bellos over at The Guardian.
So when exactly is Cheryl’s birthday, and why is the problem so frustrating for so many adults? Before we get to the solution, let’s clear up any ambiguity with the wording, which Kenneth Chang over at The New York Times calls “terrible”.
To start with, Cheryl (who we can only imagine has set her birthday to private on Facebook) will only give Albert and Bernard 10 possible dates for her birthday.
May 15, May 16, May 19
June 17, June 18
July 14, July 16
August 14, August 15, August 17
She then whispers only the month of her birthday to Albert, and nothing but the date itself to Bernard. The information Albert receives convinces him that Bernard can’t possibly know the answer.
But knowing that is enough for Bernard to figure it out. In turn, Bernard’s certainty convinces Albert of the answer, too.