Question

An 8×8×8 cm cube was painted red, and then broken up into small cubes with side lengths of 1 cm. How many small cubes have none of their faces painted red?

Answer 1

216

Explanation:

8 * 8 * 8 = 512

8 * 8 = 64

Each face is 64 cubes, overlapping at the edges, with 6 faces total.

16 + 12 = 28 for each overlapping cube on each side

64 * 6 = 384

384 - 2(28) = 328

Top & Bottom dealt with, overlap from them is 56 units total, 14 units on top and bottom of each face..

64 - 14 = 50

50 * 2 = 100

Front & Back dealt with.

328 - 100 = 228

64 - 28 = 36

36 * 2 = 72

228 - 72 = 156

...

OR

6^3 = 216

Answer 2

216

Explanation:

If you just paint the surface of the cube, then the inside of the cube would not have any of their faces painted red.

Just looking at the cube from a side view, you would realize that there would be a smaller cube, 6 x 6 x 6 (not 7 since you have to account for both the top side and the bottom side), and so that is the answer, 6 ^ 3, which is 216.