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You think you’re some kind of numbers whiz, do you?

Well then maybe you can answer a question that’s leaving people all around the world scratching their heads. Taken from a test question in Singapore, the seemingly impossible logic riddle was posted on Facebook Friday evening by “Hello Singapore” TV host Kenneth Kong.

Like most math questions you’ll remember in high school, the question derives from an awkward social situation that would never happen in real life. You have to figure out a girl’s birthday after she decides to whisper only the month to one boy and only the day to another. Because that’s a normal thing to do.

Can you solve it?

This question causes a debate with my wife …. and its a Sec 3 question. #trickquestion#cherylsbirthday

Posted by Kennethjianwen on Friday, 10 April 2015

You’re stumped, aren’t you? If it makes you feel any better, the question was written as part of the Singapore and Asian Schools Math Olympiad test, which tests the best 40 per cent of math students in the country. Organizers of the exam have also mentioned that this particular question comes toward the end of the test, and thus is aimed at only the smartest kids who managed to complete all the other questions before it.

So while it may seem impossible to answer with the limited information available, there’s actually a way to figure it out. When you’ve finally given up, scroll down for the answer and explanation.

 

 

The solution

Here’s the official explanation:

Out of the 10 dates, the day ranges from 14 to 19, with only 18 and 19 occurring once. If Cheryl’s birthday is on 18 or 19, then Bernard would have known when Cheryl’s birthday is since Cheryl has told him the day of her birthday.

But why does Albert know that Bernard does not know?

If Cheryl has told Albert that her birth month is May or June, then it is possible that her birthday may be May 19 or June 18. This means that Bernard may know when Cheryl’s birthday is. The fact that Albert knows that Bernard does not know means that Cheryl has told Albert that her birth month is either July or August.

Initially, Bernard does not know when Cheryl’s birthday is, but how did he know after Albert has first spoken?

Out of the 5 remaining dates in July and August, the day ranges from 15 to 17, with only 14 occurring twice.

If Cheryl has told Bernard the day of her birthday is 14, then Bernard would not have known. The fact that Bernard knows means the day of her birthday is not 14. So now we are left with 3 possible dates: July 16, August 15 and August 17.

After Bernard has spoken, Albert now knows when Cheryl’s birthday is. If Cheryl has told Albert her birth month is August, then Albert would not have known because there are two possible dates in August.

Cheryl’s birthday is on July 16.

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