This problem went viral across the world on 14 April 2015. It is from a test given to 14 year old students competing in the Singapore and Asian Schools Maths Olympiad, aimed at the top 40% of students.
We had a fantastic response from our community regarding the correct answer. It’s an interesting maths challenge that relies on logical thinking.
How do you approach this problem? What strategies do you use to solve it? And imagine being in a test situation too. That is an added distraction for your brain.
Do what’s the answer???
To start, Cheryl gives 10 possible dates for her birthday. Albert is then told the month and Bernard the day of Cheryl’s birthday.The challenge is to work out the actual date of Cheryl’s birthday.
Albert and Bernard at first don’t know the actual date from the information they are given. So it can’t be May 19 or June 18. Otherwise Bernard would definitely know if he had 18 or 19 as his day.
And for Albert to say that he definitely knows Bernard doesn’t know, this means it can’t be any date in May or June (when these two numbers 18 and 19 appear) either.
So that leaves July or August.
But then Bernard says he does know when the date of Chery’s birthday.
It can’t be July 14 or August 14 as then Bernard would not know which month is the correct one.
So that leaves July 16, August 15 or August 17.
And then Albert says he knows now too. That means there is no double up.
It can’t be August 15 or 17 as then Albert wouldn’t know which day to select.
It must be July 16.