The 20 cm cube can be sliced into four(a) layers - each one 20 cm wide, 20 cm long and 5cm high.
Each of those layers can be sliced into four(b) strips 20 cm wide, 5 cm long and 5 cm high
Each of those strips can be sliced into four(c) cubes 5cm wide, 5 cm long and 5 cm high
Four(a) x four(b) x four(c) = 64
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How many smaller cubes are not painted at all if a cube is painted green on all sides & cut into 64 cubes of equal size?
8
1
8
Cube root of 64 is 4 therefore cube is 4 x 4 x 4. Each side of the cube has 16 visible small cube faces, 4 of which are corners (3 green sides) and 8 more are edges (2 green sides), so there are only four small cubes with one green face on each of the six sides of the large cube. Your answer is therefore 24.