Maths question for kids goes viral on the net

A mathematics problem originally meant for schoolchildren has taken the internet by storm after it was posted online by a Singaporean television presenter.

The problem, which might seem confusing in the beginning, was however, very doable and had attracted thousands of comments and shares, The Independent reported on Monday.

"Hello Singapore" presenter Kenneth Kong, who made the post online clarified that it was actually a problem set to 14-year-olds in the Singapore and Asean Schools Math Olympiads (SASMO).

Here is the problem, which has prodded the grey cells of people:

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 and 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 it either.

Bernard: At first I din'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?

After Kong's post evoked such an interest, he was contacted by Henry Ong, the executive director of SASMO, who clarified that the "supposedly P5 (primary school) question" was posed to mathematics Olympiad contestants on April 8.

Ong also stated that it was "a difficult question meant to sift out the better students". According to Ong, he was nonetheless pleased "that this question has generated so much interest and 'solutions' on the internet".

And here's the true "solution", which emerged out of a debate in Singapore's Study Room:

First, we have 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 certain that Bernard does not know the birthday, and 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 does not 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 July 16, August 15, or August 17 (not August 14 as then he can not know).

Since Albert subsequently can also be sure of the date, he must know it is July. If it is August, he cannot be sure, as there are two dates, August 15 and 17.

So the answer is July 16.