Well...the stupid easy answer would be that there could be just 1 item in common across all 55 cards, with only the position of that object changing from card to card, with 385 other random objects distributed across the cards. That would make for a pretty boring game though, if you figured out that there's an apple on every card, for instance, because then you'd just have to look for the apple.
There's got to be a better answer though, but I may not be smart enough to figure it out
Cindy