George built the figure shown using seven unit cubes. how many such cubes does he haze to add to the figure to make a cube with the edges of length 3?

Explanation:

Think of a Rubik's cube. Each face is composed of 9 small squares, in which the goal is to get each side to have the same color of 9 squares.

Since we have 9 squares on one face, this means we have each side that is sqrt(9) = 3 units long. Each little square is 1 unit by 1 unit.

The volume of the Rubik's cube is 3*3*3 = 27 cubic units. Meaning that we need 27 small cubes to build up the entire device. We can think of it like saying there are 9 cubes in the top row, 9 in the middle row, and 9 in the bottom row to give 9+9+9 = 27 total cubes.

This figure shows 7 such cubes, so we need 27-7 = 20 more cubes to form a complete Rubik's cube.