i dont see how one is sufficient and 2 is not sufficient..

what was their reasoning for st1 being sufficient?

Crap... I fixed it... it should have been 2^16 instead of 216.

They basically factored it out for S1:
=2^8 - 1
=(2^8 + 1)(2^8 - 1)
=(2^8 +1)(2^4 +1) 2^4 -1)
=.... eventually they factored into an expression (2^2-1) which equals to 3... meaning is a multple of 3 and thus xy is div by 3.

I dunno, I didn't factor it out... I guessed that whatever answer you get is gonna tell you if XY is div by 3... so I though S1 was sufficient... WRONG thinking, b/c if y wasnt a multiple of 3, then it would be insufficient.

[/img]

ha ok that makes much more sense... but still agree with you on statement two... it should be sufficient as well in my mind....