Cheryl and her birthday

Calendar with ten possible birthdays markedProbably this year’s most-talked-about logic puzzle concerns a birthday which is not explicitly revealed, but in that familiar fashion where just sufficiently many hints are given to reveal the information if you think about the clues and their implications.

The puzzle’s Singaporean origin is much discussed, generally making glowing and reverent reference to the nation’s world-renowned education system; at first, the puzzle was claimed to be a question asked of 11-year-olds, but a Singaporean site provides photographic evidence that it’s question 24 out of 25 (and, traditionally, the hardest questions are at the end) of a Singapore and Asian Schools Math Olympiad paper. This is aimed at the top 40% or so of students aged 14 or 15. Does that make you feel less bad about it? Not so much here, either…

The Guardian newspaper reports on the question and inspires over four thousand comments in the answers. They kindly follow up by posting a worked solution. A second answer proves popular in the comments and another post at the Guardian, two days later, argues in its favour.

If you’re curious, the answer I came up with was the second one, but I was convinced by the argument that the first one proves correct. For the argument from the definitive source, see the executive director of the contest’s clarification. Good enough for me; I was wrong.

Other than that, it’s interesting to see the world report on the reporting, as this post does once all the hoo-ha has died down. This site enjoyed the take at with a cute tale at the bottom and also coverage at the aperiodical which is the most comprehensive of the lot, not only with links to discussions in favour of the various arguments, many with videos, but also with links to several other puzzles at a variety of levels of formality and difficulty if you want to explore the field in further detail.

If this leaves you in the mood for more puzzles, the Escape Reviewer site of the Greater Toronto Area is running a contest starting at noon Eastern time (5pm UK time) today, in collaboration with Escape Games Review and Escape Room Addict who produced a puzzle hunt between them last month. There are scores of exit game prizes of interest to people nearby, but the puzzles should entice solvers worldwide. The previous puzzle hunt’s solutions are a work of art and feature ringing endorsements from known puzzle hunt veterans, though when they say “not too hard” there perhaps needs to be some… context applied from who’s being quoted.

Anyway, this leaves the world looking forward to this contest; as all the puzzles are being rolled out at once, rather than on a daily basis, perhaps that’s something to keep the world occupied this weekend!


  1. On another site I frequent, this related puzzle was floated:
    Alec is either 29,36,45,47 or 54 years old. He emails the sum of the digits from his age to Bob and the product of the digits from his age to Charlie. Following this exchange:
    Charlie: I do not know his age.
    Bob: Now I know his age.
    Charlie: Now I also know his age.

    1) How old is Alec ?
    2) If Alec motivates them with “you can do it!” (informs them beforehand that they will be able to find his age in this manner), would they both still be able to find his age?

    Whilst the basic form of the first question is pretty much the same as the Birthday one (although perhaps both more elegant in construction and more clumsy in execution), the fun part comes in considering the second question which perhaps initially seems pointless but becomes more interesting as you think about it.


Leave a Comment.