Module 2 · SSC CGL Reasoning
Venn Diagram
Venn Diagram · Set Theory · Logical Venns, and open dice folding.
Venn Diagram · Set Theory · Logical Venns · Open Dice
Learning Objectives
- Understand cube basics and painted cube formulas
- Apply techniques to solve dice problems using opposite/adjacent face rules
- Practice with exam-level questions to build speed and accuracy
Key Concepts
1. Cube Basics
A cube is a 3D solid with 6 faces, 12 edges, and 8 vertices. In SSC CGL reasoning, cubes are often painted (on some or all faces) and then cut into smaller cubes. You need to count how many small cubes have 0, 1, 2, or 3 faces painted.
Fig 1: Basic cube structure
Key Cube Facts: Faces = 6, Edges = 12, Vertices = 8. When a cube of side n (n small cubes per edge) is cut after painting:
- Total small cubes = n³
- Cubes with 3 faces painted (corners) = 8 (always, regardless of n, as long as n ≥ 2)
- Cubes with 2 faces painted (edges) = 12(n-2)
- Cubes with 1 face painted (faces) = 6(n-2)²
- Cubes with 0 faces painted (core) = (n-2)³
Tip 1: Memorize the Painted Cube Formulas
3 faces painted = 8 (corners, always). 2 faces = 12(n-2). 1 face = 6(n-2)². 0 faces = (n-2)³. Verification: 8 + 12(n-2) + 6(n-2)² + (n-2)³ = n³. Always sum to verify!
Tip 2: Different Paint Patterns
SSC CGL varies paint patterns: all 6 faces painted, only 5 faces (one face not painted), opposite faces painted, adjacent faces painted. Adjust formulas accordingly. If 1 face is unpainted, subtract contributions of that face.
Example: Painted Cube
A cube of side 5 is painted on all faces and cut into 1×1×1 cubes. How many small cubes have exactly 1 face painted?
a) 48 b) 54 c) 60 d) 72
a) 48 b) 54 c) 60 d) 72
Solution: b) 54. Formula: 6(n-2)² = 6(5-2)² = 6×9 = 54. Verification: 3-face=8, 2-face=12(3)=36, 1-face=54, 0-face=27. Total = 8+36+54+27 = 125 = 5³.
Tip 3: n < 2 Cases
If n = 1 (a single cube): all 6 faces are part of the same cube. 3-face painted doesn't apply as there are no separate corner cubes. If n = 2: no interior (0-face) cubes: (2-2)³ = 0. Cubes with 1 face: 0. All 8 cubes are corner cubes with 3 faces painted.
Tip 4: Two-Face Paint — Edge Cubes
Cubes on the edges (excluding corners) have 2 faces painted. Each edge has (n-2) such cubes. There are 12 edges. So 12(n-2). For n=5: each edge has 3 cubes with 2 faces painted, total = 12×3 = 36.
2. Painted Cubes — Variations
SSC CGL often modifies the painting pattern. Common variations include: different colours on different faces, only some faces painted, or cubes painted with stripes/diagonals.
Variation 1 — Different Colours on Opposite Faces: Three pairs of opposite faces have different colours. Cubes with 2 faces painted of different colours exist on edges where two coloured faces meet.
Variation 2 — Only 5 Faces Painted: One face is not painted. Cubes on that face have 1 less painted face than expected. Adjust: subtract the face's contribution from each category.
Variation 3 — Only 3 Adjacent Faces Painted: Three faces meeting at one vertex are painted. Only 1 corner has 3 faces painted (the common vertex).
Example: Different Colours
A cube of side 4 is painted red on top-bottom, green on left-right, and blue on front-back. It is cut into 1×1×1 cubes. How many have exactly two different colours?
a) 24 b) 20 c) 16 d) 12
a) 24 b) 20 c) 16 d) 12
Solution: a) 24. Cubes on edges where two differently coloured faces meet. There are 12 edges, each with (n-2)=2 cubes. But each edge connects two faces of specific colours. All edges have different colour pairs. So 12×2=24 cubes with 2 colours. Cubes with 3 colours = 8 (corners).
Tip 5: Two-Colour Edge Cubes
For different colours on faces: each edge connects two faces. Cubes on that edge (excluding corners) have exactly those two colours. Count = 12(n-2) for any two-colour combination. For specific colour pairs, divide by the number of edges with that pair.
3. Dice Problems — Standard Dice
A standard die (dice) has numbers 1 to 6 on its faces, arranged so that the sum of numbers on opposite faces is 7. SSC CGL tests your ability to find opposite faces, adjacent faces, and identify valid/open dice configurations.
Standard Die Rule: Opposite faces sum to 7: 1↔6, 2↔5, 3↔4.
Adjacent Faces Rule: Two faces that share an edge are adjacent. If two numbers appear on the same die in different views, they are adjacent (not opposite).
Fig 2: Standard die opposite face pairs
Tip 6: Opposite Face = 7 - Given Face
For a standard die, if you see one face, the opposite is 7 minus that number. Face 3 → opposite is 4. Face 6 → opposite is 1. This works ONLY for standard dice where opposite faces sum to 7.
Example: Dice Opposite Face
In a standard die, if face 2 is on top, what is at the bottom?
a) 2 b) 5 c) 4 d) 3
a) 2 b) 5 c) 4 d) 3
Solution: b) 5. Opposite of 2 = 7-2 = 5. Standard die: 1↔6, 2↔5, 3↔4.
Tip 7: Adjacent Faces Rule — Finding Opposite
If two dice show different views, find a common number in both views. The numbers adjacent to the common number in one view, when combined with the other view, help identify the opposite face. Method: if face A is adjacent to B, C, D, E in two views, then the remaining face (not seen) is opposite to A.
Example: Finding Opposite from Two Views
In two views of a standard die, View 1 shows [Top: 1, Front: 2, Right: 3] and View 2 shows [Top: 6, Front: 2, Right: 4]. Which number is opposite to 2?
a) 1 b) 5 c) 3 d) 6
a) 1 b) 5 c) 3 d) 6
Solution: b) 5. In View 1: 2 is adjacent to 1 and 3. In View 2: 2 is adjacent to 6 and 4. So 2 is adjacent to 1,3,6,4. The only remaining number is 5, which must be opposite to 2.
4. Open Dice — Net Folding
An open die (or dice net) is a 2D layout of 6 squares that can be folded into a cube. SSC CGL asks you to identify which nets form a valid cube, or which faces will be opposite when folded.
Valid Dice Nets: There are 11 distinct nets that fold into a cube. The most common are the cross shape (6 squares in a T or cross pattern), the staircase pattern, and the zigzag pattern.
Opposite Face Rules in Nets:
- In a cross net, the four side squares form a ring — opposite pairs are separated by one square in between
- Squares separated by exactly one square (with a row/column gap) become opposite faces
- If two squares share an edge in the net, they are adjacent faces in the cube
Fig 3: Open die net — cross pattern, 1 and 6 are opposite (separated by 3)
Tip 8: Opposite Faces in Nets — The Gap Rule
In a net, if two squares have exactly one square between them along a row or column (with no turns), they become opposite faces when folded. Example: in a cross, the top square and bottom square (with the center in between) are opposite.
Tip 9: Adjacent Faces in Nets
Squares sharing an edge in the net are adjacent faces. Squares that share only a corner (not an edge) are not adjacent — they may be opposite or diagonal. Trace the folding path to verify adjacency: squares that fold toward each other become adjacent.
Example: Open Die Net
In a dice net cross pattern with numbers: top=1, middle row=2-3-4-5, bottom=6. Which two numbers are opposite?
a) 1 and 4 b) 1 and 6 c) 2 and 5 d) 3 and 4
a) 1 and 4 b) 1 and 6 c) 2 and 5 d) 3 and 4
Solution: b) 1 and 6. In the cross pattern, the top (1) and bottom (6) squares are separated by the center square (3) — they become opposite when folded. Also, 2 and 4 are opposite, and 3 and 5 are opposite... Actually, let me recheck with the standard cross net. 1 is top, 2-3-4-5 are middle row, 6 is bottom. When folding: 1 folds over 3 (becomes opposite to 6). 2 and 4 fold to become opposite. So 1↔6, 2↔4, 3↔5.
Tip 10: Folding Technique — Trace and Match
Mentally fold the net by lifting the sides. Choose one square as the base. Fold adjacent squares up. Visualize which squares meet at edges and corners. With practice, you can identify opposite/adjacent pairs in 10-15 seconds without physical folding.
Tip 11: SSC CGL Common Dice Traps
Trap 1: A number shown on a die that cannot exist (e.g., face with number 7). Trap 2: Two views that show contradictory adjacent relationships. Trap 3: Open die nets that are invalid (squares overlapping when folded). Always verify that the net has exactly 6 squares and no overlapping when folded.
Tip 12: Dice Rotation Method
When comparing two views of a die, keep one common face fixed and rotate mentally. If face X appears in both views, and face Y is adjacent to X in view 1 while face Z is adjacent in view 2, then Y and Z are opposite each other (both adjacent to the same face X).
Example: Dice Rotation
View 1: Top=A, Front=B, Right=C. View 2: Top=A, Front=D, Right=E. Find the opposite of B.
a) C b) D c) E d) A
a) C b) D c) E d) A
Solution: b) D. A is common. In View 1, B and C are adjacent to A. In View 2, D and E are adjacent to A. Since rotating around A shows different faces, the faces not visible together are opposite. B and D appear on different views in the front position — they are opposite.
SSC CGL Shortcut Techniques
Tip 13: Painted Cube — Quick Sum Check
Always verify: 8 + 12(n-2) + 6(n-2)² + (n-2)³ = n³. If your numbers don't sum correctly, recalculate. This catches most errors immediately.
Tip 14: Standard Dice — Opposite by Elimination
If given a dice with faces showing numbers, list all adjacent pairs from each view. The number that never appears adjacent to a given face must be its opposite. This works for both standard and non-standard dice.
Tip 15: Open Die — Valid Net Check
A valid dice net has exactly 6 squares and can be folded without overlapping. Quick check: in the net, no square should share edges with more than 4 squares (since a cube face has 4 adjacent faces). Count edge connections.
Solved Examples
Solved Example 1 (Painted Cube — Basic)
A cube of side 6 is painted on all faces and cut into unit cubes. How many small cubes have at least 2 faces painted?
a) 56 b) 64 c) 48 d) 72
a) 56 b) 64 c) 48 d) 72
Solution: a) 56. 3-face = 8, 2-face = 12(6-2) = 48. At least 2 faces = 8+48 = 56.
Solved Example 2 (Painted Cube — 5 Faces)
A cube of side 4 is painted on 5 faces (one face left unpainted) and cut into unit cubes. How many cubes have 0 faces painted?
a) 12 b) 14 c) 16 d) 18
a) 12 b) 14 c) 16 d) 18
Solution: b) 14. With all 6 faces: (4-2)³ = 8 interior cubes. But one face is unpainted, so the layer of cubes on that face that would normally have 1 painted face now have 0. That face has (4-2)² = 4 cubes. Total 0-face = 8+4 = 12. Wait — the interior is 8, and the unpainted face adds its face-layer cubes (excluding edges which are already counted)... Actually the face has n² = 16 cubes. The edge cubes (12) and corners (4 on that face) are already accounted. The face-centre cubes = (n-2)² = 4. These are interior to that face but on the surface. Normally they have 1 face painted. With unpainted face, these 4 cubes have 0 faces painted. So total 0-face = 8 (core) + 4 = 12. hmm let me recalculate. Answer should be 12. But given option a is 12, that's correct. Actually let me verify option numbers: a)12 b)14 c)16 d)18. 12 is a.
Solved Example 3 (Dice — Opposite Faces)
In a standard die, three views show: (1) Top=2, Front=3, Right=6; (2) Top=4, Front=3, Right=1; (3) Top=5, Front=2, Right=1. Find the number opposite to 1.
a) 3 b) 4 c) 5 d) 6
a) 3 b) 4 c) 5 d) 6
Solution: c) 5. From (2): 1 is adjacent to 4 and 3. From (3): 1 is adjacent to 5 and 2. So 1 is adjacent to 4,3,5,2. The only remaining number is 6, so 1 and 6 are opposite. Wait, 1↔6. But option d is 6. Let me recheck. Options: a)3 b)4 c)5 d)6. If 1 is adjacent to 4,3,5,2 then opposite must be 6. Answer d) 6.
Solved Example 4 (Dice — Adjacency)
Two positions of a die are shown: Position 1: Top=3, Front=5, Right=2. Position 2: Top=1, Front=5, Right=4. Which number is opposite to 5?
a) 6 b) 3 c) 1 d) 2
a) 6 b) 3 c) 1 d) 2
Solution: a) 6. 5 appears in both views. In Pos 1: 5 adjacent to 3 and 2. In Pos 2: 5 adjacent to 1 and 4. So 5 adjacent to 3,2,1,4. Only 6 is missing → 5 opposite to 6.
Solved Example 5 (Open Die)
In a dice net, the squares are arranged as: Row1: _1_, Row2: 2-3-4-5, Row3: _6_. Which number is opposite to 3?
a) 1 b) 2 c) 4 d) 6
a) 1 b) 2 c) 4 d) 6
Solution: a) 1. Cross pattern. 3 is the centre of the cross. When folded, 1 (top) folds over 3 and meets 6 (bottom). 3 is adjacent to 2,4,5, and 1. Actually 3 is opposite to 1... let me think again. In the cross: 1 is top, 2 left, 3 centre, 4 right, 5 bottom-right? No: Row2: 2-3-4-5. 2,3,4,5 are in a row. 1 above 3, 6 below 3. When folded: 3 is the base. 1 folds to become top (opposite 3). 6 folds to become... no. Let me visualize: Fold 2 and 4 up (they become sides). Fold 1 down over 3 (it becomes the face opposite 3). Fold 5 and 6... 6 folds under 3. So 1 is opposite 3. Answer: a) 1.
Solved Example 6 (Painted Cube — 3 Adjacent Faces)
A cube of side 3 is painted on three adjacent faces only and cut into unit cubes. How many have exactly 1 face painted?
a) 6 b) 7 c) 8 d) 9
a) 6 b) 7 c) 8 d) 9
Solution: b) 7. With 3 adjacent faces (meeting at one corner), each painted face has (n-2)² = 1 face-centre cube. So 3 cubes with 1 face painted from face-centres. Additionally, cubes on the edges (excluding the common corner) between two painted faces have 2 faces painted, not 1. But wait — some cubes on the border of a painted face only have that one face painted (because the adjacent face is not painted). On each painted face, the edge cubes that border an unpainted face have only 1 face painted. Counting carefully: each painted face shares edges with 2 other painted faces (2 edges) and 1 unpainted face (1 edge). The edge cubes on the unpainted side have 1 face painted. Each such edge has (n-2) = 1 cube. 3 faces × 1 such edge × 1 cube = 3. Plus face-centre cubes: 3. Total with exactly 1 face: 3+3 = 6. But the corner common to all 3 painted faces has 3 faces painted. The other 3 corners on the painted faces have 2 faces each. So the count should be... Let me reconsider. 3 adjacent faces painted means 3 faces that meet at one vertex. The 3 faces are like the top, front, and right of a cube. Face-centre of each = 1 cube (total 3). Cubes on the edges where painted meets unpainted: each painted face has 1 edge that borders an unpainted face (the edge opposite to the common vertex). Each such edge has (n-2) = 1 cube with exactly 1 face painted. 3 such edges → 3 more. Total = 6. But option a is 6. Hmm but there could be the cubes at the centre of each face that are single-faced. Actually, let me be more precise. For n=3: (n-2)=1. Face-centre cubes = 1 per face = 3 total. Then edge cubes where painted meets unpainted: each painted face has one such edge with (n-2)=1 cube with 1 face painted. 3 edges → 3. Total = 6. So answer a) 6.
Solved Example 7 (Dice — Sum of Visible Faces)
A standard die is placed on a table. What is the sum of the numbers on all visible faces?
a) 15 b) 16 c) 17 d) 18
a) 15 b) 16 c) 17 d) 18
Solution: c) 17. Total of all faces = 1+2+3+4+5+6 = 21. Bottom face (hidden) can be 1 to 6 depending on the top face. Assuming any valid standard die placement, one face is on the bottom (not visible), one on top is visible, and 4 side faces are visible. Sum of all 6 = 21. Sum visible = 21 - bottom face. If top = x, bottom = 7-x. Visible = 21 - (7-x) = 14+x. For top = 3: visible sum = 17. Different tops give different sums. The question likely implies a specific arrangement where sum of visible = 17 (top=3, opposite=4, visible faces = top+4 sides = 3+2+4+5+6... actually if top=3, bottom=4, visible = top + 4 sides (which are 1,2,5,6 in some order) = 3+1+2+5+6 = 17).
Solved Example 8 (Painted Cube — Two Faces)
A 5×5×5 cube is painted green on two opposite faces and cut into unit cubes. How many cubes have at least one green face?
a) 50 b) 45 c) 40 d) 55
a) 50 b) 45 c) 40 d) 55
Solution: a) 50. Two opposite faces painted. Each face has n² = 25 cubes. But edge cubes on the 4 common edges of these faces are counted twice. Each edge has n = 5 cubes. 4 edges share between the two faces, so overlap = 4×5 = 20. Total = 25+25-20 = 30. But wait, corner cubes (4 corners per face) are counted in both faces, but the subtraction handles that through edge overlap. Actually each of the 4 edges has n cubes (including corners). The two faces share 4 edges. 2 faces × 25 = 50. Subtract the shared edge cubes counted twice: 4 edges × 5 = 20. 50-20 = 30. But actually only the cubes on those two faces are painted. 25+25 = 50 total painted-face cubes. The edge overlap is already within the same face. Each face has its own edges. They are separate faces (opposite), their edges don't overlap. So total unique cubes with at least one green face = 25+25 = 50. But wait, no cubes are on both opposite faces (they're opposite!). So there's no overlap. 25+25 = 50. Answer a) 50.
Solved Example 9 (Dice — Open Die Net)
Which of the following nets cannot form a closed cube?
a) Cross of 6 squares b) T-shape with 4 in a row and 2 attached c) 5 squares in a row with 1 attached d) A row of 6 squares
a) Cross of 6 squares b) T-shape with 4 in a row and 2 attached c) 5 squares in a row with 1 attached d) A row of 6 squares
Solution: d) A row of 6 squares. A 1×6 strip cannot fold into a cube — two squares would overlap. Valid nets: cross (a), T-shape (b), and staircases. A row of 6 is invalid.
Solved Example 10 (Painted Cube — Colours)
A cube of side 4 is painted red on two adjacent faces, blue on two other adjacent faces, and left unpainted on the remaining two faces. How many cubes have no paint?
a) 28 b) 24 c) 20 d) 16
a) 28 b) 24 c) 20 d) 16
Solution: a) 28. Total cubes = 64. Painted area: 4 painted faces (2 red + 2 blue) each with 16 cubes, but edge/ corner overlaps. 4 faces × 16 = 64. Subtract edges shared between painted faces (12 edges of n=4 = 48 cubes counted multiple times)... This is complex. Simpler: count unpainted cubes directly. Unpainted faces are opposite each other (since the 4 painted faces wrap around). The unpainted region includes all cubes that don't touch painted faces. The core (n-2)³ = 8. Plus cubes on the unpainted faces (2 faces × (n-2)² = 8). But these face-cubes on the unpainted side are NOT painted. Plus the edge cubes between the two unpainted faces: 4 edges × (n-2) = 8. Plus the 4 corners where the two unpainted faces meet = 4. Total unpainted = 8+8+8+4 = 28. Answer a) 28.
Solved Example 11 (Dice — Number Arrangement)
On a standard die, if 1 is on top and 2 is in front, what is on the right face?
a) 3 b) 4 c) 5 d) 6
a) 3 b) 4 c) 5 d) 6
Solution: a) 3. Standard die: opposite pairs = (1,6), (2,5), (3,4). Top=1, bottom=6(opposite). Front=2, back=5(opposite). The remaining faces are 3 and 4. On a standard die, when 1 is on top and 2 is in front, the right face is 3 (standard orientation).
Solved Example 12 (Painted Cube — Ratio)
For a cube of side n painted on all faces, what is the ratio of cubes with 2 faces painted to cubes with 1 face painted?
a) 2:(n-2) b) 2n:(n-2) c) 2:(n-2)² d) (n-2):2
a) 2:(n-2) b) 2n:(n-2) c) 2:(n-2)² d) (n-2):2
Solution: a) 2:(n-2). Ratio = 12(n-2) : 6(n-2)² = 12 : 6(n-2) = 2 : (n-2).
Summary of SSC CGL Venn Diagram Shortcuts
- Painted Cube: 3-face=8, 2-face=12(n-2), 1-face=6(n-2)², 0-face=(n-2)³, Total=n³
- Standard Die: Opposite faces sum to 7. 1↔6, 2↔5, 3↔4
- Open Die Nets: Valid nets have exactly 6 squares and fold without overlap. Opposite faces are separated by 1 square in the net
- Adjacent Faces Rule: If a face appears with different adjacent faces in two views, those adjacent faces (from different views) are opposite each other
- Dice Opposite by Elimination: List all adjacent faces; the missing number is opposite