When is Cheryl’s birthday? How to figure out the Singapore math problem confusing the Internet

WATCH ABOVE: The Morning Show crew tried to tackle the ‘Cheryl’s birthday’ riddle that’s taken the internet by storm.

TORONTO – So when is Cheryl’s birthday? That’s a question confounding thousands of people around the world Monday as a test question from Singapore goes viral.

The test question asks students to use logic and limited information to figure out Cheryl’s birthday. The question gives ten dates and tells Cheryl’s friends Albert and Bernard separately the month or the day of her birthday, respectively.

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The question was posted by Kenneth Kong, a TV anchor in Singapore and has since been shared thousands of times.

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The question is part of the Singapore and Asian Schools Math Olympiad test which is given to some of the best students in the country. Those same students would be in about Grade 10 in Canada.

But the question is meant to be difficult. Kong posted a letter allegedly written by Henry Ong, the executive director of the organization which administers the test, which said the question is the second last question on the 25 question test and it was “meant to sift out the better students.”

So can you solve it? Give it a try before reading further and getting the answer.


How to figure out Cheryl’s birthday

“It’s definitely quite a curious question, it’s not a typical high school question,” Dr. Alexey Kuznetsov, an associate professor of mathematics at York University said in an interview. “It’s a very, very nice question, I quite enjoyed it myself.”

The solution involves using logic to deduce the dates which can’t possibly be Cheryl’s birthday.

The dates range from 14 to 19 among the 10 that are given with only 18 and 19 occurring once. Albert, having seemingly been told the month rather than the day, first says he doesn’t know when her birthday is – eliminating both 18 and 19 as possible days.

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If Cheryl had told Albert that the month was May or June, then the day could have been May 19 or June 18, and Bernard may have known the right day. But, as the question says, Albert knows Bernard does not, meaning that Cheryl has said her birthday is in either July or August.

Out of the five remaining days in July and August, the day ranges from 14 to 17, with 14 appearing twice.

If Cheryl told Bernard her birthday was on the 14th, then he would not have known but, he does, meaning it can’t be on the 14th.

That leaves only 3 possible days: July 16, August 15, and August 17.

After Bernard speaks, saying he knows the birthday given that information, it eliminates August from being a contender since he still wouldn’t have known whether it was August 15 or 17.

Therefore, Cheryl’s birthday is on July 16.