You are fitting 3 by 5 inch cards into a box and some cards are side by side... Hence, the dimensions of 2 piles side by side is 6 x 5. So you have a square box in which you can fit an object of dimension 6 x 5, then the box must be at least 6 x 6 x 6. Unless I'm misinterpreting. It does seem quite ambiguous.