.
There is nothing easier to solve.
a) How many 1cm cubes have painted on 3 faces?
Those and only those that are located at the vertices (corners) of the 3x3x3 cube: exactly 8 small cubes.
b) How many 1cm cubes have painted on 2 faces?
Those and only those that are located at the edges but not attached to vertices: exactly one at each edge; 12 small cubes in all.
c) How many 1cm cubes have painted on 1 face?
Those and only those that are located at the center of each face and not attached NEITHER to vertices NOR edges:
exactly one at each face; 6 small cubes in all.
d) How many 1cm cubes are not painted at all?
Exactly 1 (one, ONE) located at the center of the 3x3x3 cube.
Check. 8 + 12 + 6 + 1 = 27 = 3*3*3. ! Correct !