Video

# When Is Cheryl’s Birthday?

## Solving a logic problem that spread around the world.

When, exactly, is Cheryl’s birthday? A charming logic puzzle led to much hand-wringing over this question in the past week. A prompt about two men, Albert and Bernard, trying to figure out a woman’s birth date with limited information spread from a Singapore math competition to social media around the world. Below, read the original puzzle, then watch our video explanation of the solution.

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?

Here’s how to figure it out, with one simple chart:

And if you are the type of person who would rather be told the answer than shown, here’s a bonus way to think about it for Clue players, courtesy of Paul Smith, a developer for Slate’s Panoply:

The solution becomes clear when you translate Cheryl’s clues into Clue-speak:

May = Library

June = Conservatory

July = Dining Room

August = Ball Room

14 = Mrs. White

15 = Col. Mustard

16 = Professor Plum

17 = Miss Scarlet

18 = Mr. Green

19 = Mrs. Peacock

Suspects seen in …

The Library: Col. Mustard, Professor Plum, Mrs. Peacock

The Conservatory: Miss Scarlet, Mr. Green

The Dining Room: Mrs. White, Professor Plum

The Ball Room: Mrs. White, Col. Mustard, Miss Scarlet

(Albert knows the room and Bernard knows the killer.)