It's more of a question than a riddle

I have 4 piles of cards, respectively containing 8,9,7 and 8 cards, the composition ain't important, but i need to know where I must add a cart for it to make a coherent cards combination. Doesn't really have to make sense, I'll just accept the first answer as true. I realize it's not really the goal of this site but I really need an answer. Thank you.