A cube painted red on all faces is cut into 27 identical smaller cubes. How many small cubes have three faces painted?
Correct answer: A. 8
Only the 8 corner cubes have three painted faces, whatever the size of the cube.
Solved reasoning MCQs on painted cubes cut into smaller cubes, the minimum number of cuts, cube facts and standard dice, with the formulas that solve them.
This set covers painted cubes cut into smaller cubes, counting cubes with paint on three, two, one or no faces, the minimum number of cuts needed, the edges and faces of a cube, and the numbers on opposite faces of a standard die.
For a cube cut into n × n × n pieces, corners always give 8 cubes with three faces painted, and the inside gives (n − 2)³ with none. The explanations show each formula worked out.
Correct answer: A. 8
Only the 8 corner cubes have three painted faces, whatever the size of the cube.
Correct answer: B. 12
Cubes with two painted faces lie on the edges but not at corners: 12 × (n − 2) = 12 × 1 = 12.
Correct answer: C. 6
One painted face means the centre of each face: 6 × (n − 2)² = 6 × 1 = 6.
Correct answer: A. 1
The unpainted cubes form the inner core: (n − 2)³ = 1³ = 1.
Correct answer: A. 24
Here n = 4 (4 × 4 × 4 = 64). Two faces painted: 12 × (n − 2) = 12 × 2 = 24.
Correct answer: B. 24
Here n = 4. One face painted: 6 × (n − 2)² = 6 × 4 = 24.
Correct answer: C. 8
Here n = 4. No face painted: (n − 2)³ = 2³ = 8.
Correct answer: D. 36
Here n = 5 (5 × 5 × 5 = 125). Two faces painted: 12 × (n − 2) = 12 × 3 = 36.
Correct answer: A. 54
Here n = 5. One face painted: 6 × (n − 2)² = 6 × 9 = 54.
Correct answer: B. 27
Here n = 5. No face painted: (n − 2)³ = 3³ = 27.
Correct answer: C. 96
Here n = 6 (6 × 6 × 6 = 216). One face painted: 6 × (n − 2)² = 6 × 16 = 96.
Correct answer: D. 64
Here n = 6. No face painted: (n − 2)³ = 4³ = 64.
Correct answer: A. 48
Here n = 6. Two faces painted: 12 × (n − 2) = 12 × 4 = 48.
Correct answer: B. 98
Total 125 minus the 27 unpainted inner cubes (3³) = 98.
Correct answer: C. 6
Two cuts in each of the three directions divide every edge into 3 parts: 3 × 2 = 6 cuts.
Correct answer: D. 9
Each edge must be split into 4 parts, which needs 3 cuts in each of the three directions: 3 × 3 = 9 cuts.
Correct answer: A. 12
A cube has 6 faces, 12 edges and 8 vertices (corners).
Correct answer: C. 4
3 + 4 = 7. The opposite pairs are 1–6, 2–5 and 3–4.
Correct answer: D. 21
1 + 2 + 3 + 4 + 5 + 6 = 21, which is also three opposite pairs of 7.
Correct answer: A. 15
The hidden faces are opposite the visible ones: 5 (opposite 2), 6 (opposite 1) and 4 (opposite 3); 5 + 6 + 4 = 15.