Math Problem For Singapore Students Stumps The Internet — Can You Solve It?

Even if math isn’t a strength, surely most adults could solve a math problem designed for 14-year-old students, right?


A math question designed for 14-year-old students located in Singapore was posted on the Facebook page of famous presenter of Singapore TV Kenneth Kong — and it quickly went viral, showing the world just why Singapore generally ranks among the top countries in in primary maths performances.

Are you ready to be stumped?

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?

Sounds like Cheryl is just being mean, right?

If you’re stumped, don’t feel too badly. Although the math question is designed for teenagers, it was first directed towards students participating in the Singapore and Asian School Math Olympiads (SASMO) — and for a country who ranks highly in student math accomplishments, these students are the best of the best. Furthermore, it was question number 24 out of 25 math problems specifically designed to separate the very strongest of math students from the rest, says Henry Ong, executive director of SASMO.

But instead of just sifting out the strongest from the weakest for the math competition, the question has managed to stump the majority of the world, as well.

“We are not saying this problem is for every student… But if these kind of problems can be used to stretch the better students to sharpen their analytical power, why not?” Hong says.

Why not indeed? But those who wrestled unsuccessfully with the logical difficulty offered up other solutions — such as giving Cheryl roses and asking her when her birth date was again, only nicely — or dumping her, and finding a different girl.

If you’re ready for the answer, supplied by the obvious math geniuses of the Singapore Study Room, keep reading.

First we need to figure out if Albert knows the month or the day. If he knows the day, then there is no chance that Bernard knows the birthday, so it must be that Albert knows the month.

From the first statement, we know that Albert is sure that Bernard doesn’t know the birthday, so May and June should be ruled out (the day 19 only appears in May and the day 18 only appears in June). In other words, if Albert had May or June, then he cannot be sure that Bernard doesn’t know, since Bernard could have had 18 or 19.

Following that statement, Bernard knows that May and June are ruled out.

Then, Bernard is able to know which month it is. So it must be either July 16, August 15 and August 17 (not 14th as then he can’t know).

Since Albert subsequently can also be sure of the date, he must know it’s July. If it’s August, he can’t be sure as there is August 15 and 17.

So the answer is July 16.


Let us know in the comments below — did you make an effort to solve the math problem yourself? If so, were you successful?

For another brain teaser, this one visual rather than mathematical, click here.

[Photo by Marco Di Lauro / Getty Images]