Originally Posted by
dspguy
Either the cups are 90° separated or 180°. Pick one and note if the other cup is "ahead" or "behind" from the starting position. Then whenever it finishes, you can tranpose where the 2nd cup was. For example, if they started North and East and you follow the east one... then the second cup was "behind" (since they always rotate clockwise). If the watched cup stops at south, then you know the unwatched cup must be east. So, the safe spot is northwest.
Obviously, this isn't for Infindox - just for another person reading it that might need a more detailed explanation :)