Richard and Jack just become friends with Harry, and they want to know when is his birthday. Harry gives them a list of ten possible dates.
|May 15||May 16||May 19|
|June 17||June 18|
|July 14||July 16|
|August 14||August 15||August 17|
Harry then tells Richard and Jack separately the month and the day of his birthday respectively.
Richard: I don't know when Harry's birthday is, but I know Jack doesn't know too.
Jack: At first I don't know when Harry's birthday is, but I know now.
Richard: Then I also know when Harry's birthday is.
When is Harry's birthday?