Mathematics Professor Pierre Marchand at Univ of Nancy, France
told me this one.
There are two integers x,y between 2 and 100, inclusive, and
two people P,S. P is told the product x*y. S is told the
sum x+y. Neither is told the specific values of x,y.
P,S have the following conversation:
S: I cannot determine x,y.
P: I cannot determine x,y.
S: I already knew that.
P: In that case, I now know x,y.
S: In that case, I too now know x,y.
What are x,y? (and how long did it take to solve?)