We’ll start with an easier version of this challenge that uses cards with three attributes (instead of four). Each card in this “3-dimensional SET®” has a shape, a number and a color: For instance, one card would be “2 red ovals.” The number is 1, 2 or 3; the color is red, green or purple; and the shape is oval, diamond or squiggle. So there are 27 cards in all.