What’s Cheryl’s birthday? How to solve Singapore’s viral maths problem

A school maths question posted on Facebook by a Singaporean TV presenter has gone viral, stumping thousands worldwide.

The initial post said that it was originally set for a Year 5 test for Singaporean kids, which caused many to throw up their arms in surrender.

Fortunately, it was later revealed that the problem was in fact set for a maths olympiad test for high school-age whizz kids, which made everyone feel a little better. But for most of us, only a little. 

The question asks students to guess the birthday of a girl called Cheryl using clues – or to many of us odd meaningless phrases – given by her friends, Albert and Bernard.

Here is the question (the English is a little untidy, but you will get the gist of it ):

Albert and Bernard just become friends with Cheryl, and they want to know when her birthday is.
Cheryl gives them a list of possible dates.

May 15 May 16 May 19
June 17 June 18
July 14 July 16
August 14 August 15 August 17

Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.

Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard does not know too.

Bernard: At first I don’t know when Cheryl’s birthday is, but I know now.

Albert: Then I also know when Cheryl’s birthday is.

So when is Cheryl’s birthday?

Give up? If you do, maths author and columnist Alex Bellos is here to help in the BBC video above.

You can read more by Alex Bellos in Comsos: Celebrating pi – our favourite non-whole number and Seduced by calculus.

Bill Condie

Bill Condie

Bill Condie is a science journalist based in Adelaide, Australia.

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