This is a logic test that was part of the Singapore and Asian Schools Math Olympiad – a competition for teenagers.
Not everybody can agree on what the answer is.
IT'S SO CONFUSING.
So what is the answer – and how the hell do you work it out? Here's our explanation – if you want to try working it out for yourself, stop scrolling now.
OK, here goes...
Here's what we know: Albert is told only the month. Bernard is told only the day.
When he's told what the month is, Albert immediately works out that Bernard doesn't know the correct answer.
What does that mean? It means that the answer can't be a number that only occurs once in the 10 options. If the date was 18 or 19, Bernard would know the correct answer straight away, because there's only one of each (May 19 and June 18).
So how can Albert, who only knows the month, be so sure Bernard cannot possibly know this? Because he knows the month isn't May or June.
So we can cross all of May and June off the list of possibilities.
Now, Bernard speaks up. He confirms he didn't know what the answer was…but says that, after Albert's answer, he now does know.
How is that possible? It's because he's followed Albert's logic, and has worked out that the answer isn't May or June.
What can we work out from the fact that Bernard now knows for sure what the answer is? One thing: It means that it can't be a 14, because that day's repeated across both months.
For Bernard to know the answer, it has to be one of the unique days – which means it must be July 16, August 15, or August 17.
So let's scratch July 14 and August 14 off the list.
But hang on: Once Bernard says he does know, now Albert suddenly knows the answer too!
Remember, he only knew the month – but he's also followed Bernard's logic, so he (like us) knows it's one of three possible days.
So what can we work out from this? Well, if the month was August, Albert still couldn't know what the answer was, because there are two possible days in that month left. So the fact that Albert does know means one obvious thing...
CHERYL'S BIRTHDAY IS ON JULY 16!
Well done, Albert and Bernard! (In case you don't believe us, the actual answer sheet is on mothership.sg.) And in fairness, the creators of the test told mothership.sg that "being Q24 out of 25 questions, this is a difficult question meant to sift out the better students". So don't feel too bad if you didn't get it right.