Build a sugar-cube pyramid as follows: First make a 5 × 5 × 1 bottom layer. Then center a 4 × 4 × 1 layer on the first layer, center a 3 × 3 × 1 layer on the second layer, and center a 2 × 2 × 1 layer on the third layer. The fifth layer is a single 1 × 1 × 1 cube. Express the volume of this pyramid as a percentage of the volume of a 5 × 5 × 5 cube.