Sagiro
Rodent of Uncertain Parentage
Vyper said:Pielorinho, you're on the right track! I'm impressed to get such a quick and good reply. Honestly, it took me much longer to get as far as you got.
Unfortunately 2 and 9 is not the answer.
As you showed, if the sum is 11, A can be certain that B cannot find out the numbers. (It isn't the only number with this property. 17, for example, is another one.)
Now, if A tells B that he knows that B cannot find out the numbers, B knows, that 9 and 2 must be the answer, since he got 18, and that can only be factored as 2*9 or 3*6. 3 and 6 is impossible, because 3+6=9, and that can also be the sum of the primes 2 and 7 (if both numbers are prime, B would immediately know them from the product). So B knows the numbers.
But ...
It can't be 30, since 30 can also be factored 15*2 (as I said before, 17 is another number that allows A's statement). But 18, 24 and 28 are all possible factors, so A cannot know the numbers at this point.
So, 2 and 9 is not the correct solution.
As far as I can tell, this line of reasoning narrows the possible products down to ten possibilities:
18 (2, 9)
24 (3, 8)
28 (4, 7)
50 (2, 25)
52 (4, 13)
54 (2, 27)
76 (4, 19)
92 (4, 23)
96 (3, 32)
98 (2, 49)
...but I'll be darned if I can figure out how B is able to narrow it down further without additional information.
That last step's a doozie!

-Sagiro