Nobody guessed 4. I could create the 5x5 cube with 4 rectangular blocks (2x1x1, 3x2x1, 4x3x1, 5x5x1). I could do it with less if we didn't assume any kind of specific shape to the block, as few as one block, formed to the negative space. The volume of the single block would still be 45 x (L/5)^3, assuming L is the distance along one edge of the 5x5 cube.