The maths problem that’s gone viral: When the hell is Cheryl’s birthday?

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.

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4 Responses to The maths problem that’s gone viral: When the hell is Cheryl’s birthday?

  1. lawrie says:

    The answer is 19 of May.
    When looking for a sequence of numbers you will
    note that each date is separated by either 1 or 2 days.
    The odd one being 19th of may which is separated by
    3 days,
    Do you agree 🙂

  2. lawrie says:

    We can discount the 19th of May and the 18th of June
    as these 2 dates only appear in in one month and it would
    be immediately apparent to Bernard which month Cheryls
    Birthday would be in.
    This would leave June the 17th as the logical choice as it
    is the only single choice for any given month but this again
    would let Bernard easily know which month the birthday is in.
    So we exclude this date and we are left with the following.
    May 15th May 16th May 19th
    June 17th June 18th
    July 14th July 16th
    Aug 14th Aug 15th Aug 17th
    Which would leave Aug 17th as the logical date
    as it is the only single choice left.

    Failing this then I would have left out this question in the exam
    and gone on to achieve a 99% score 🙂 🙂 🙂

  3. lawrie says:

    Or then again I could have just cheated
    and got a top score of 100% by going to this link
    Cheryl’s Birthday on Wikipedia

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