2. Calculate the blank area of the upper left corner: 4× 4+6× 6-1/2× 6× 4+6) = 22;

Find the area of the blank part in the lower right corner: 1/2×5×(4+6+5)=37.5.

3. The area of the painted part in three squares: 4× 4+5× 5+6× 6-22-37.5 =17.5.

4. Coloring area: 2.5+ 17.5=20.