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.
Puzzle: 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?
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.
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.