The solution, explained like you’re 5

[UPDATE 6:33 p.m: According to Singapore and Asian Schools Math Olympiad (SASMO) Executive Director Henry Ong, the question is NOT a Primary 5 test question, but from a Math Olympiad contest for Sec 3 and 4 students.]

[UPDATE 5 May 7:30p.m.]

Here’s the model answer. Previously we did not attribute the source of model answer, but this image has since been attributed to mothership.sg. The screenshot came from here first.

solutioncherylbirthday

Source

So most of you would probably have seen that blasted Primary 5 Math Olympiad question floating around online:

https://www.facebook.com/kennethjianwenz/photos/a.173663129479243.1073741827.167504136761809/385751114937109/?type=1

Everyone’s first thought: Wow, this Cheryl is evil as hell. What kind of “friend” forces people to guess her birthday with such twisted questions?

Is this really a question for Primary 5 students?

Apparently so not.

Some people have already figured out the answer, but the conundrum certainly left a lot of people scratching their head (and cursing this Cheryl).

We’ve managed to figure out the real answer, and just so you can show off to your friends, here’s the result!

If you don’t know the answer yet, read on.

The long-awaited solution:

First, we have to establish the information which each of our characters know.

Albert: Knows only the birth month

Bernard: Knows only the birth date

Cheryl: Knows both day and month

These are the 10 possible dates that Cheryl gave:

stupidmath

Because the statements provided are really hard to understand, we took the liberty of changing the words around instead.

1. 

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

Edited statement: Albert: I am confident that Bernard does not know when Cheryl’s birthday is.

Sounds like a game of high-stakes poker.

Okay, so Bernard knows Cheryl’s birth date, but he does not know the month.

Albert knows that Bernard does not know her birthday. This is because the birth date (the number) has to be repeated in other months, otherwise Bernard would have figured out her birthday immediately.

The months of May and June are out because the two months have dates which do not repeat in other months (18/19), which means Cheryl’s birthday has to take place in either July or August.

(Cells in red have been eliminated as possibilities)

stupidmath4

Source

2. 

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

Edited statement: Bernard: I wasn’t sure before, but with the added info Albert provided, I can deduce her birthday now.

Bernard knows that the birthday does not occur in May or June.

He also knows Cheryl’s birthday based on the date he was given.

14 cannot be the date because Bernard would have two months to choose from (14 July and 14 August), thus making him unsure.

Because Bernard is sure of Cheryl’s birthday, therefore, her birthday can only fall on 16 July, 15 August, or 17 August.

At this point, Bernard knows the birthday is 16 July because of the number he received, but he hasn’t told Albert this yet.

stupidmath5

Source

3. 

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

Edited statement: Albert: If Bernard deduced that her birthday cannot fall on the 14th, I can deduce her birthday too.

Bernard knows the date and eliminated July 14 and August 14, so Albert can also eliminate these two dates.

stupidmath6

Source

Since Albert now knows Cheryl’s birthday from eliminating July 14 and August 14, it cannot be either 15 or 17 August (as there is more than one possible answer if August was the month); therefore July 16 is the only possible answer.

Hence, both of them now know Cheryl’s birthday.

The answer is July 16.

Still confused? Watch this video for another explanation.

Yes, this is a Primary 5 Math question

If even adults can’t solve this problem, what hope is there for 11-year-old students to solve it on their own? I certainly don’t remember problems being this hard in primary school. Without proper guidance from the teacher, this question appears to be excessively difficult and could stress children out even more.

Regardless, now that we have the answer, we have to applaud the logic and deductive thinking involved in the question, although we also have to question the sanity of the setter in giving this question to Primary 5 students. That’s just downright evil, man.

Also, the English was far from perfect and could hinder the ability of students to understand the question. Someone commented that if “know” in the question was replaced with “deduced”, the question would have been more easily understood.

What other crazy questions are children being bombarded with today, and will such difficult questions lower the self-esteem of students further? Boy, am I glad I didn’t have to deal with these questions when I was in Primary 5…I’d probably quit doing Maths for the rest of my life.

#CherylIsEvil #SoIsTheTestSetter

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Featured image via Kennethjianwen
With references from Hardware Zone, Hardware Zone, Mothership