Viral Singapore Math Problem

Viral media has gradually become central to mainstream conversation as a result of our increasing reliance on social media. Usually, this takes the form of something which instantly elicits short-term enjoyment – whether it is a video of an unruly commuter, a bizarre photo of a cat, or a Miss Universe contestant answering a question with as many non-words in 30 seconds or less. So when a logic question from a Singaporean Maths Olympiad started to make the rounds on social networks, one might question why something so complex and niche has the capacity to be shared or viewed by millions of social media users.

Here’s the puzzle below. Click here for the solution and an explanation.

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 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?

So, why did something as seemingly dry as deductive logic captivate the internet? Here are a few possible reasons for this:

  • Originally, it was claimed the puzzle was taken from a Primary 5 (Year 6) maths test. It was later revealed to be taken from a Maths Olympiad, but in any case, many internet users grabbed the opportunity to provide social commentaries about the rigorousness of the Singaporean education system.
  • The deceiving simplicity of the puzzle. There are many facets of the puzzle which the reader may assume to be simple:
    • The context of guessing a birthday with a month and a day does not appear to be very involved.
    • There does not appear to be a huge amount of information to sort through – but, counterintuitively, it is this scarcity of information which causes the most confusion.
    • The reader is presented with options, which would naturally be perceived as being easier in the same way multiple choice questions can appear more solvable than short answer questions.
    • The language is plain and does not allude to any complex mathematical operations. Even in the final solution, the most complex technique required is process of elimination.
  • Constructing a pathway to solve the puzzle involves some lateral thinking which the reader may not be expecting:
    • Selection of detail in the question is critical. Albert’s seven innocent-looking words “I know that Bernard does not know” instantly eliminate half of the possibilities.
    • Organising the logic appropriately into plain English can actually become quite exhausting. Bernard’s second statement can be very easily interpreted as “I know the solution because I know that Albert knew that I had not known the solution earlier”. But, by organising the logic appropriately, this should be read as “Bernard knows the answer, because he knows the month is July or August”.

In any case, it was somewhat refreshing to see a logic puzzle get some airtime on the social media waves as opposed to a picture of a cat wearing a tuxedo. Unfortunately, my attempt to create a viral image of my CT8: Financial Economics exam only garnered genuine confusion amongst my followers.

The solution and explanation of the puzzle can be viewed here.

Were you able to solve the puzzle, and if so, did you take a different route to the solution?

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