Peter Norvig
May 2015

# When BK Met Eve: A Birthday Story The *Cheryl's Birthday* logic puzzle [made the rounds](https://www.google.com/webhp?#q=cheryl%27s+birthday), and I wrote [code](Cheryl.ipynb) that solves it. In that notebook I said that one reason for solving the problem with code rather than pencil and paper is that you can do more with code. **[Gabe Gaster](http://www.gabegaster.com/)** proved me right when he [tweeted](https://twitter.com/gabegaster/status/593976413314777089/photo/1) that he had extended my code to generate a new list of dates that satisfies the constraints of the puzzle: January 15, January 4, July 13, July 24, July 30, March 13, March 24, May 11, May 17, May 30 In this notebook, I verify Gabe's result, and find some other variations on the puzzle. First, let's recap [the puzzle](https://en.wikipedia.org/wiki/Cheryl%27s_Birthday): > 1. Albert and Bernard became friends with Cheryl, and want to know when her birthday is. Cheryl gave 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 > 2. **Cheryl** then privately tells Albert the month and Bernard the day of her birthday. > 3. **Albert**: "I don't know when Cheryl's birthday is, and I know that Bernard does not know." > 4. **Bernard**: "At first I don't know when Cheryl's birthday is, but I know now." > 5. **Albert**: "Then I also know when Cheryl's birthday is." > 6. So when is Cheryl's birthday?