Earlier this year, the Internet was rocked by the viral sensation that was “The Dress,” a seemingly magical, changing-color-before-your-eyes gold-and-white/blue-and-black bandage dress that literally achieved world wide attention (and obsession). Now, a fictional woman named Cheryl with a mysterious birthday is enjoying that same break-the-Internet brand of fame (sorry, Kim K).

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This math problem first came about during a math olympiad competition in Singapore held on April 8. Unbeknownst to these teenaged math geniuses, the problem they were given was about to go viral. Singaporean television personality Kenneth Kong posted a picture of the problem on his Facebook page, and in a matter of days, #CherylsBirthday was trending.

So…are you smarter than a Singaporean high schooler? Let’s find out:


 

***SPOILERS*** if you want to tackle the problem yourself, don’t scroll down any further!

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Let’s rewrite the problem so it makes a little bit more sense:

Albert and Bernard have just become Facebook friends with Cheryl, but Cheryl doesn’t have her birthday listed on her Facebook page (well, Cheryl, don’t complain when nobody writes on your wall). Albert and Bernard, eager to start preparing birthday photo collage posts for Cheryl, ask when her birthday is. Cheryl gives them 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 the month of her birthday and Bernard the day of her birthday–because it’s more fun for Cheryl that way.

Albert says, “I don’t know when Cheryl’s birthday is, but I know that Bernard does not know, either.”

Bernard says, “At first, I didn’t know when Cheryl’s birthday was, but now I know when her birthday is.”

Albert says, “Now I also know when Cheryl’s birthday is!”

Cheryl says, “Good, then I can be expecting presents from both of you??”

It seems really complicated at first glance, but once you break it down it’s actually pretty simple.

1) Albert says, “I don’t know when Cheryl’s birthday is, but I know that Bernard does not know, either.”

Albert doesn’t know when Cheryl’s birthday is, which makes sense–whether it was May, June, July, or August, there would still be at least two options for each month. But he also knows that Bernard doesn’t know, which means Cheryl’s birthday is not in one of the months which features a date that only shows up once–May (19 only shows up once) or June (ditto 18). So Cheryl’s birthday is either in July or August.

July 14                 July 16                

August 14           August 15        August 17

2) Bernard says, “At first, I didn’t know when Cheryl’s birthday was, but now I know when her birthday is.” 

Before Albert spoke, Bernard didn’t know when Cheryl’s birthday was. But now, Bernard knows (like us) that Cheryl’s birthday is in either July or August. If Cheryl had told Bernard her birthday was on the 14th, then he would not know–it could be either July 14 or August 14. But Bernard says he does know, which means Cheryl’s birthday is one of the following:

July 16

August 15        August 17

3) Albert says, “Now I also know when Cheryl’s birthday is!”

There are two options left for August, but only one option for July. So if Cheryl’s birthday were in August, Albert still wouldn’t know when her birthday is–it could be the 15th or the 17th. That means JULY 16 MUST BE CHERYL’S BIRTHDAY!!!! feeling ~victorious~

4) Cheryl says, “Good, then I can be expecting presents from both of you??”

The last part of the puzzle: what will Albert and Bernard get Cheryl for her birthday???

a) the white-and-gold/blue-and-black dress

b) a calendar

c) a locked box with a present inside that only opens if you solve a very difficult brain teaser

d) nothing, after making them go through all this trouble just to find out when her birthday was