X and Y just became friends with Robert. They want to know his birthday. So, he gave them 10 possible dates to choose from.
May 15, May 16, May 19
June 17 ,June 18
July 14 July 16
August 14, August 15, August 18
Then Robert tells X and Y month and day of his birthday respectively.
X: I don't know his birthday but I know Y also doesn't know
Y: Earlier, I did not know, but now I know.
X: Now I know it too.
Assuming both X and Y are perfectly logical,When is Robert's Birthday?