When Is Cheryl's Birthday? Maths Question Goes Viral

A fiendishly difficult maths question from an exam for students at an elite Singapore high school has gone viral.

Posted on the Facebook wall of the Singapore and Asian School Math Olympiads (SASMO) it involves two children being asked to guess the birthday of their ten-year-old friend Cheryl.

The full text is as follows:

Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 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 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?

First of all why is Cheryl being so mysterious? What ten-year-old does that?

Secondly, here are some of our favourite (non)-answers from the interwebs:

Matthew Bayley wrote: “My answer would be to stop hanging round with Cheryl. She’s obviously very annoying.” [We're going with that one too.]

“Spending my birthday trying to figure out when Cheryl’s birthday is. Albert and Bernard won’t tell me,” tweeted TKeezy with a mournful air.

Meanwhile, Kellie volunteered: “Genuinely do not care when Cheryl’s birthday is and is she was that bothered she would tell Albert and Bernard the actual date.”

To avoid a mass brain bleed, SASMO has mercifully put us out of our collective miseries and given us the following model answer:

It also clarified that the question was not, as initially reported posed to primary school children, rather from the Secondary 3 and from the Secondary 4 SASMO contests for 15/16 year-olds and was defined as “a difficult question meant to sift out the better students.”

HOWEVER... since the answer was published, several people have claimed there is an alternative answer to the question, placing Cheryl's birthday on 17 August. SASMO has offered the following explanation as to why this is not the case. We remain non the wiser.