Here's The Answer To The Cheryl's Birthday Math Problem

It's one thing for celebrity hijinks or cute animal stories to take off on social media. That happens all the time.

It's not too often a math problem goes viral, especially when it doesn't involve a complaint about Common Core.

Over the weekend, Singapore TV personality Kenneth Kong posted a logic problem on his Facebook page that was given to high school kids competing in a math olympiad.

The problem reads:

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?

Kong wrote that the problem had caused a debate with his wife, and they're hardly alone. People argued about the answer on Facebook and Twitter with a passion you don't usually see over math.

"The worldwide excitement and curiosity about this problem is very encouraging," Steven Strogatz, professor of applied mathematics at Cornell University said via email. "It shows that people love to think logically (or at least, to try to think logically), just for the pleasure of it. Even though the problem isn't good for anything, it's still fun to think about."

While the question burned up bandwidth and got people excited, a logic problem may not be the best way to increase the understanding of mathematics.

“It’s a fun problem, though in the end just a puzzle," Jordan Ellenberg, math professor at the University of Wisconsin and author of "How Not to Be Wrong," told The Huffington Post via email. "A really deep math question would be to ask students to *construct* a puzzle like this.”

So what's the answer?

For a highly detailed look at the answer, check out this explanation from The Guardian.

If you were stumped by the question, don't feel too bad. It was meant to be hard.

“Being Question 24 out of 25 questions, this is a difficult question meant to sift out the better students,” Singapore and Asian School Math Olympiads wrote on Facebook. “SASMO contests target the top 40% of the student population and the standards of most questions are just high enough to stretch the students.”

And, it seems, plenty of adults.

The Huffington Post's David Freeman contributed to this report.



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