Cheryl entrusts the month to one and the day to another. When both speak, they do not describe what they know: they reveal, unintentionally, what they do not know. That's enough to find the date.
Cheryl's birthday
Riddle statement
Albert and Bernardo have just met Cheryl. She tells them that her birthday is one of these ten dates:
- May 15, May 16, May 19
- June 17, June 18
- July 14, July 16
- August 14, August 15, August 17
Later, Cheryl secretly tells Albert the month and Bernardo the day.
Then this conversation occurs:
- Albert: “I don't know when Cheryl's birthday is, but I know Bernardo doesn't know either.”
- Bernardo: “At first I didn't know, but now I know.”
- Albert: “Then I know it too.”
What is Cheryl's birthday?