Non-Verbal & Visual Reasoning

Non-verbal reasoning asks you to think in shapes instead of words — mirror images, folded paper, dice, painted cubes, hidden figures. In the real question paper these come as pictures. This chapter cannot print pictures, so it does something more useful: it teaches you the rule behind each type, using letters and digits (whose shapes you already know), numbered dice faces, and figures described exactly in words. Learn the rule here and you will be able to apply it to any picture the examiner throws at you.

Mirror Images

ℹ️ How to read this chapter
Every example below is solvable in words alone. Where a topic genuinely needs a drawing — a figure series, an embedded figure — this chapter does not fake a picture. It gives you the checking rule instead, so that in the exam hall you know exactly what to look at first. That is the part students actually get wrong.

A mirror stands on the vertical axis — usually to the right or the left of the object. So it swaps left and right, and leaves top and bottom exactly as they were. Nothing goes upside down. That one sentence solves most mirror-image questions.

Capitals and digits that are symmetric about the vertical axis — they look UNCHANGED in a mirror
GroupMembersWhy
Capital letters (11)A, H, I, M, O, T, U, V, W, X, YTheir left half is the mirror of their right half
Digits (2)0, 8Both are built from closed loops balanced left and right
Everything elseB, C, D, E, F, G, J, K, L, N, P, Q, R, S, Z and 1–7, 9Reverses left-to-right, so it looks laterally inverted
🧠 Memory trick
Do not memorise eleven scattered letters. Notice the pattern: A, H, I, M, O and then an unbroken tail T, U, V, W, X, Y. Say it as "AHIMO + TUVWXY". Add 0 and 8 for digits. Six words that stay perfectly readable in a mirror: MOM, WOW, TOOT, MUM, TOT, HAH — all palindromes made only of AHIMO-TUVWXY letters.

For a whole word two things happen together, and students usually remember only one. First, the order of the letters reverses (the last letter now sits nearest the mirror). Second, each individual letter is laterally inverted — unless it is one of the symmetric letters above. Both effects apply at once.

  1. MOM → reverse the order: M-O-M, still MOM (it is a palindrome). Each letter M, O, M is a symmetric letter. So the mirror image reads MOM, completely unchanged.
  2. BOOK → reverse the order: K-O-O-B. Now check each letter: K and B are not symmetric, so both appear laterally inverted; the two O's are unchanged. So the mirror shows the letters in the order K, O, O, B with the K and B flipped.
  3. MAXIMUM → every letter (M, A, X, I, M, U, M) is symmetric, so no letter changes shape; but the order still reverses, so the mirror reads MUMIXAM. This is the classic half-mark trap.
  4. HIT 108 → reverse everything: 801 TIH. H, I, T, 0, 8 are all symmetric; only the 1 flips. So the shapes barely change, but the reading order is fully reversed.

Water Images

A water image is the reflection you see in a still pond. The mirror now lies on the horizontal axis, below the object. So it swaps top and bottom, and leaves left and right exactly as they were. Crucially, the order of the letters does NOT reverse — the word still reads left to right.

Capitals and digits that are symmetric about the horizontal axis — they look UNCHANGED in water
GroupMembersWhy
Capital letters (9)B, C, D, E, H, I, K, O, XTheir upper half is the mirror of their lower half
Digits (4)0, 1, 3, 8Balanced above and below the middle line
Compare with the mirror listOnly H, I, O (and 0, 8) are in BOTH listsThese few survive a mirror image AND a water image
🧠 Memory trick
The water list has its own pattern: the run B, C, D, E at the start of the alphabet, then H, I, K, O, X. Say "BCDE + HIKOX". Digits: 0, 1, 3, 8. And remember the overlap — H, I, O, 0, 8 are the true survivors of both kinds of reflection.
  1. BOOK → order stays B-O-O-K. Now check each letter against the water list: B ✓, O ✓, O ✓, K ✓ — all four are symmetric about the horizontal axis. So the water image of BOOK reads BOOK, completely unchanged.
  2. HIDE → H, I, D, E are all on the water list, order unchanged, so the water image reads HIDE unchanged. Its mirror image, however, reads EDIH with the E and D flipped.
  3. 1308 → all four digits (1, 3, 0, 8) are on the water list, so the water image reads 1308 unchanged. But the mirror image reverses the order to 8031, with the 1 and 3 flipped.
  4. MOM → the reverse case. In a mirror MOM is unchanged, but in water M is not symmetric about the horizontal axis, so each M turns into a W-like shape: the water image looks like WOW, not MOM.
⚠️ Mirror vs water — the same word, two different answers
Take BOOK. Its water image is BOOK (unchanged — B, O, O, K are all horizontally symmetric and the order is kept). Its mirror image is K, O, O, B (order reversed, K and B laterally inverted). Same word, opposite answers. So before you touch the options, read the question again and settle one thing: is the reflection on the vertical axis (mirror → reverse the order) or the horizontal axis (water → keep the order)? Students who skip this step lose the mark even when they know both letter lists perfectly.

Paper Folding, Punching and Cutting

The whole family of paper questions rests on one idea: a punch cuts every layer it passes through, so the number of holes equals the number of layers pierced. To find the answer, do not try to imagine the finished sheet in one jump. Work backwards, unfolding one step at a time, and at each unfolding double the holes by reflecting them across that fold line.

Because each fold doubles the layers, n folds turn a single punch into 2ⁿ holes — provided the punch misses every fold line. So 1 fold → 2 holes, 2 folds → 4 holes, 3 folds → 8 holes. Two punches with 2 folds give 4 + 4 = 8 holes.

  1. Worked case 1 — one fold. Take a square sheet and think of it as four quadrants: top-left, top-right, bottom-left, bottom-right. Fold the right half exactly onto the left half, so the crease is the vertical centre line and the packet is two layers thick. Now punch once, high up in the middle of the packet, clearly away from the crease.
  2. Answer to case 1: 2 layers pierced → 2 holes (= 2¹). Unfold and reflect across the vertical centre line: one hole sits in the top-left quadrant and its twin in the top-right quadrant, both at the same height from the top edge and at the same distance from the centre line.
  3. Worked case 2 — two folds. Start again with the square. Fold 1: right half onto left half (vertical crease at the centre). Fold 2: bottom half onto top half (horizontal crease at the centre). The packet is now one quadrant — say the top-left — and is four layers thick. Punch once near the middle of that quadrant, on neither crease.
  4. Answer to case 2: 4 layers pierced → 4 holes (= 2²). Unfold the horizontal crease first: the hole in the top-left quadrant gains a twin in the bottom-left quadrant. Now unfold the vertical crease: those two are reflected into the top-right and bottom-right quadrants. Final picture: one hole in each of the four quadrants, all equally far from the vertical centre line and equally far from the horizontal centre line — the four holes are the corners of a rectangle centred on the middle of the sheet.
⚠️ A punch sitting ON a fold line does not double
The 2ⁿ rule assumes the punch is clear of every crease. If the punch straddles a fold line, unfolding that fold produces one larger hole spanning the crease, not two separate holes. Example: fold twice as in case 2, but punch right on the second (horizontal) crease. Unfolding fold 2 gives one hole sitting astride the horizontal centre line; unfolding fold 1 then doubles it. Total = 2 holes, not 4. Always check the punch's position against the creases before you multiply.

Paper cutting follows the same logic, with one extra rule about where the cut is made. A cut that touches the fold line unfolds into a single symmetric shape straddling the fold. A cut made on a free edge (away from the fold) unfolds into two mirror-image notches. This is exactly how children make paper snowflakes.

  1. Worked cutting case. Fold the square's right half onto its left half. The packet's right-hand edge is the crease; its left-hand edge holds the sheet's original left and right edges lying on top of each other. Halfway up the crease, snip off a small triangle whose base lies along the crease.
  2. Unfold it: the triangle is doubled across the crease, so the two halves join into one diamond-shaped (rhombus) hole sitting exactly on the vertical centre line, at mid-height of the sheet. One cut, one hole — not two.
  3. Now change only the position. Cut the same triangle from the packet's left-hand edge instead. Unfolding gives two separate triangular notches — one on the sheet's left edge and one on its right edge, at the same height, mirror images of each other.
  4. Two folds, one cut at the centre corner. Fold the square twice (vertical, then horizontal). The corner of the packet where the two creases meet is the centre of the sheet. Snip that corner off and unfold: you get a single hole at the dead centre, symmetric about both centre lines — four quarters of the cut joined into one shape.

Cubes & Dice — Finding Opposite Faces

A cube has 6 faces, 12 edges and 8 corners. Any single view of a cube can show at most three faces, and those three faces meet at one corner — so they are mutually adjacent. From this comes the golden rule: any two numbers seen together in the same view can never be opposite each other. Every dice question is solved by ruling out neighbours until only one candidate is left.

Two-view case: two faces are common
PositionVisible faces
Position I2, 3, 5
Position II2, 3, 6

Here 2 and 3 are common to both views, and only the third face changes (5, then 6). Reason it out: the faces 2 and 3 share one edge, and that edge has two ends. At one end the third face is 5; at the other end it must be the face opposite to 5. Since the second view shows 6 at that corner, 6 is opposite 5. General result: when two views share two common faces, the two odd faces are opposite each other — and the two common faces are adjacent to both.

Three-view case: hunt for the number that meets four others
PositionVisible facesWhat it proves
Position I1, 2, 31 is adjacent to 2 and 3
Position II1, 3, 41 is adjacent to 4 as well
Position III1, 4, 51 is adjacent to 5 as well

The number 1 has now been seen with 2, 3, 4 and 5 — that is four different neighbours, and a face has exactly four neighbours. So the only number left, 6, is opposite 1. Finish the die: 3 has been seen with 2 and with 4, so 3 cannot be opposite either — therefore 3 is opposite 5, and the last pair is 2 opposite 4. Full answer: 1–6, 2–4, 3–5. Notice the method: find the number with four known neighbours first; the rest falls out by elimination.

The ordinary (standard) die convention: opposite faces add up to 7
FaceOpposite faceSum
167
257
347
⚠️ Two dice traps that cost easy marks
(1) Adjacent is not opposite. If a view shows 2 and 5 together, then on that cube 2 and 5 are neighbours — so it is not an ordinary die and the sum-to-7 shortcut does not apply. Use the common-face method instead. (2) Only use "sum = 7" when the question says the die is an ordinary or standard die. When the paper simply gives you views of a cube with numbers on it, derive the pairs from the views; do not import the 7 rule. Also remember: a cube view can show three faces at most, so any option claiming four visible faces is wrong by construction.

The Painted Cube

A cube of side n units is painted on all six faces and then cut into n³ unit cubes. How many little cubes have 3, 2, 1 or 0 painted faces? Do not count — think about where a small cube sits in the big one. Its position decides how many of its faces were on the outside.

🧠 The cube itself tells you the formulas
Do not memorise four formulas — read them off the cube. A cube has 8 corners, 12 edges, 6 faces. A small cube at a corner has 3 painted faces → always 8. A small cube on the middle of an edge has 2 painted faces, and each edge holds (n−2) middle cubes → 12(n−2). A small cube in the middle of a face has 1 painted face, and each face holds (n−2)² middle cubes → 6(n−2)². Everything hidden inside has 0 painted faces, and the inside is itself a cube of side (n−2) → (n−2)³. So: 8, 12×, 6×, inside — the corner-edge-face-interior story.
Painted-cube formulas, with n = 4 and n = 5 worked out
Painted facesWhere it sitsFormulan = 4 (64 cubes)n = 5 (125 cubes)
3 facesAt a corner8 (always)88
2 facesMiddle of an edge12(n−2)12 × 2 = 2412 × 3 = 36
1 faceMiddle of a face6(n−2)²6 × 4 = 246 × 9 = 54
0 facesHidden inside(n−2)³2³ = 83³ = 27
Total checkAll small cubesn³8+24+24+8 = 64 ✓8+36+54+27 = 125 ✓
  • Always run the total check. The four counts must add up to n³, because every small cube is either a corner, an edge-middle, a face-middle or an interior cube. For n = 4: 8 + 24 + 24 + 8 = 64 = 4³ ✓. For n = 5: 8 + 36 + 54 + 27 = 125 = 5³ ✓. If your four numbers do not sum to n³, you have made an arithmetic slip — fix it before choosing an option.
  • "At least one face painted" = n³ − (n−2)³ (everything except the hidden interior). For n = 4: 64 − 8 = 56. For n = 5: 125 − 27 = 98. Read the question carefully: exactly one face is 6(n−2)², while at least one is this much bigger number.
  • Smallest case, n = 3 (27 cubes): 8 corners with 3 faces, 12 × 1 = 12 with 2 faces, 6 × 1 = 6 with 1 face, and 1³ = 1 with no face — the single cube at the very heart. 8 + 12 + 6 + 1 = 27 ✓. If n = 2, there is no interior at all: all 8 small cubes have 3 painted faces.

Counting Figures Without Missing Any

Counting questions are lost to carelessness, not difficulty. Never scan the figure and guess. Use a fixed order every time: count the smallest units first, then figures made of two units, then three, and so on, and finally the whole figure. Add the groups at the end. A second, foolproof check for triangles: list every set of three labelled points and keep only those where all three joining lines are actually drawn.

  1. Case A — one median. Take triangle ABC. Let D be the midpoint of BC, and draw the median AD. Smallest triangles: ABD and ACD = 2. Made of two units: the whole triangle ABC = 1. Total = 2 + 1 = 3 triangles.
  2. Case B — two medians. In the same triangle ABC, let D be the midpoint of BC and E the midpoint of AC. Draw both medians AD and BE; they cross at the centroid G. The two medians cut the triangle into four regions: three triangles (ABG, BDG, AEG) and one quadrilateral (GDCE).
  3. Count Case B in groups. Made of 1 region: ABG, BDG, AEG = 3. Made of 2 regions: ABD (=ABG+BDG), ABE (=ABG+AEG), ACD (=AEG+GDCE), BCE (=BDG+GDCE) = 4. Made of 4 regions: ABC = 1. Nothing is made of exactly 3 regions. Total = 3 + 4 + 1 = 8 triangles.
  4. Verify by the point method. The eight triangles are exactly ABC, ABD, ABE, ABG, ACD, AEG, BCE, BDG. Sets like A-C-E or B-G-E fail because those three points are collinear; sets like A-D-E or C-D-G fail because the joining line (DE, or CG) was never drawn. Two methods, same answer — that is how you gain confidence in the exam.
Squares in a 3 × 3 grid (9 small cells) — count by size
Square sizeHow many positionsCount
1 × 1Every cell: 3 across × 3 down3² = 9
2 × 2Top-left corner can sit in 2 columns × 2 rows2² = 4
3 × 3Only the whole grid1² = 1
Total9 + 4 + 114 squares

The pattern generalises. An n × n grid contains n² + (n−1)² + … + 2² + 1² squares, which is the sum of the first n square numbers, n(n+1)(2n+1) / 6. Check it: for n = 3, 3 × 4 × 7 / 6 = 84 / 6 = 14 ✓, matching the table. For n = 4: 16 + 9 + 4 + 1 = 30, and 4 × 5 × 9 / 6 = 180 / 6 = 30 ✓. For a chessboard, n = 8: 64 + 49 + 36 + 25 + 16 + 9 + 4 + 1 = 204, and 8 × 9 × 17 / 6 = 1224 / 6 = 204 ✓.

🎯 Why it matters
Why bother with the formula when you could just count? Because in the exam hall you have about 40 seconds per question and no eraser-time for a recount. A remembered result (3 for one median, 8 for two medians, 14 for a 3 × 3 grid, 204 for a chessboard) turns a two-minute counting job into an instant tick — and frees your mind for the questions that genuinely need thinking.

Figure Series, Analogy & Hidden Figures

These are the question types that genuinely need pictures, so let us be honest: this page cannot draw the series for you. What it can do is give you the examiner's toolbox. Every figure series and figure analogy is built from a small, fixed set of transformations. Your job is never to "see" the answer — it is to identify the rule that turns one figure into the next, and then apply that same rule once more. Test the transformations in a set order and you will find the rule in seconds.

The examiner's transformations, and how to spot each one
TransformationHow to spot itExample you can check in text
Rotation 90° clockwiseA quarter-turn to the right: what was at the top moves to the right; verticals become horizontalsN turned a quarter-turn looks like Z
Rotation 90° anticlockwiseA quarter-turn to the left: what was at the top moves to the leftTwo anticlockwise quarter-turns = one 180° turn
Rotation 180°Upside down: top↔bottom and left↔right both swap togetherW → M, 6 → 9, b → q
Reflection (mirror)Left↔right swap only; top stays top. No amount of rotating can imitate itb → d and p → q are mirrors; b → q is a rotation
Addition / removal of an elementCount the parts in each figure of the series; a steady rise or fall of one per step gives it away2 lines → 3 lines → 4 lines → the answer has 5
Movement of a dot / small markIgnore the outline; track only the mark. Note its step size and directionDot moves one corner clockwise: top-left → top-right → bottom-right → bottom-left
Increase in the number of sidesCount the sides of the main shape in each stepTriangle (3) → square (4) → pentagon (5) → hexagon (6)
Change of shadingThe shaded part moves to the next position, or shading alternates on/offShaded, plain, shaded, plain → next is shaded
  1. Test in this order. (1) Is the outline the same shape but turned? → rotation. (2) Is it turned but the turning does not work? → reflection. (3) Has the number of parts changed? → addition or removal. (4) Has a small mark moved? → track only the mark. (5) Has shading moved? (6) Has the number of sides changed? Stop at the first rule that explains every step, not just the first step.
  2. The rule must fit every step. Beginners find a rule that explains figure 1 → figure 2 and jump to the options. Always test it on figure 2 → figure 3 as well. If it fails there, the rule is wrong, however pretty it looked.
  3. Analogy is the same skill, shortened. In A : B :: C : ?, find the single change from A to B and apply exactly that to C. It works identically for letters: AZ : BY :: CX : ? — the first letter moves one forward, the second one backward, so the answer is DW. Find the rule from the given pair; never guess from the options.

In an embedded (hidden) figure question you are given a small figure X and four larger options, and exactly one option contains X complete and unchanged. It may be rotated, but it is never stretched, shrunk or bent. Blind searching wastes a minute; a checking order finds it in fifteen seconds.

  1. Find the most distinctive feature first — an unusual angle, the one curve among straight lines, a sharp spike, a very short segment. Scan the four options only for that feature. Options that lack it are eliminated at once, without any tracing.
  2. Count the line segments of X. The correct option must contain at least that many segments arranged in the same way. If a candidate has fewer lines in that region than X needs, it is out.
  3. Verify proportions. X keeps its exact size and shape. If a candidate would need X stretched longer, squashed shorter or its angle widened, reject it — the examiner never rescales the hidden figure.
  4. Trace it with your pencil point. Follow X's outline over the surviving option. If you must lift the pencil, or invent a line that is not printed, X is not there. One clean unbroken trace = the answer.
ℹ️ Dot situations: write the dot's address
A dot situation shows several overlapping shapes and a dot placed in one region; you must find the option whose region matches. The whole trick is to stop looking at the picture and instead write the dot's "address" as a list of in/out. Example: shapes are a triangle, a circle and a square, and the dot lies inside the triangle, inside the circle, outside the square. That address — in, in, out — belongs to exactly one region of the diagram, and only one option can have it. Compare addresses, not pictures, and the question becomes mechanical. (This type does need the printed diagram; the address method is what makes it fast when you get there.)

Key takeaways

  • Mirror = vertical axis: left↔right swap, top stays; the word order reverses. Unchanged letters: A, H, I, M, O, T, U, V, W, X, Y and digits 0, 8.
  • Water = horizontal axis: top↔bottom swap, order stays. Unchanged letters: B, C, D, E, H, I, K, O, X and digits 0, 1, 3, 8. So BOOK is unchanged in water but reads K-O-O-B in a mirror.
  • Paper punching: holes = layers pierced, so n folds → 2ⁿ holes (1 fold = 2, 2 folds = 4, 3 folds = 8) — but a punch lying on a fold line does not double for that fold.
  • Paper cutting: a cut touching the fold unfolds into one symmetric shape astride the fold; a cut on a free edge unfolds into two mirror-image notches.
  • Dice: faces seen together are adjacent, hence never opposite. If two views share two common faces, the two odd faces are opposite. On an ordinary die opposite faces sum to 7 (1–6, 2–5, 3–4) — use this only when the question says the die is ordinary.
  • Painted cube of side n cut into n³ units: 3 faces = 8 (corners), 2 faces = 12(n−2) (edges), 1 face = 6(n−2)² (faces), 0 faces = (n−2)³ (interior). For n = 4: 8, 24, 24, 8 (total 64). For n = 5: 8, 36, 54, 27 (total 125).
  • Counting: go smallest units → pairs → triples → whole. Triangle with one median = 3 triangles, with two medians = 8 triangles; an n × n grid has n² + (n−1)² + … + 1² squares, so a 3 × 3 grid has 14 and a chessboard has 204.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. Which one of the following capital letters looks exactly the same in its mirror image (mirror held on the vertical axis)?

    1. B
    2. H
    3. J
    4. P
    Show answer

    Answer: B — H

    A vertical-axis mirror swaps left and right, so only letters whose left half mirrors their right half survive: A, H, I, M, O, T, U, V, W, X, Y. H is in that list. B, J and P are all lopsided left-to-right, so they appear laterally inverted. (B, however, IS unchanged in a water image, because it is symmetric top-to-bottom.)
  2. In a mirror placed on the vertical axis, the letters of the word MAXIMUM appear in which order?

    1. MAXIMUM
    2. MUMIXAM
    3. MIXAMUM
    4. MUXIMAM
    Show answer

    Answer: B — MUMIXAM

    Every letter of MAXIMUM (M, A, X, I, M, U, M) is symmetric about the vertical axis, so no letter changes shape. But a mirror still reverses left and right, so the ORDER of the letters reverses: M-A-X-I-M-U-M read backwards is MUMIXAM. Forgetting the order-reversal is the commonest error in this question type.
  3. Consider the following statements:
    1. The word MOM looks unchanged in its mirror image (vertical axis).
    2. The word BOOK looks unchanged in its water image (horizontal axis).
    Which is/are correct?

    1. Only 1
    2. Only 2
    3. Both 1 and 2
    4. Neither 1 nor 2
    Show answer

    Answer: C — Both 1 and 2

    Both are correct, for two different reasons. MOM is a palindrome, so reversing the order still gives MOM, and M and O are both symmetric about the vertical axis — hence unchanged in a mirror. BOOK is not a palindrome, but a water image does NOT reverse the order; it only flips top and bottom, and B, O, O, K are all symmetric about the horizontal axis — hence unchanged in water. Note the contrast: in a mirror BOOK reads K-O-O-B, and in water MOM looks like WOW.
  4. A square sheet of paper is folded twice — first the right half exactly onto the left half, then the bottom half exactly onto the top half. A single hole is then punched through all the layers at a point that does not lie on either fold line. How many holes are seen when the paper is fully unfolded?

    1. 2
    2. 4
    3. 6
    4. 8
    Show answer

    Answer: B — 4

    Each fold doubles the layers, so two folds give 2 × 2 = 4 layers. The punch pierces all 4 layers, and the number of holes equals the number of layers pierced, so there are 2² = 4 holes. Unfolding confirms the positions: one hole in each of the four quadrants, all at equal distance from the vertical centre line and equal distance from the horizontal centre line. (If the punch had lain on a fold line, that fold would not have doubled it and the answer would have been 2.)
  5. Two positions of the same cube are shown below, the faces being numbered 1 to 6.
    Position I: the faces 2, 3 and 5 are visible.
    Position II: the faces 2, 3 and 6 are visible.
    Which number is on the face opposite to 5?

    1. 6
    2. 3
    3. 2
    4. 1
    Show answer

    Answer: A — 6

    The two views share exactly two common faces, 2 and 3, so the two odd faces are opposite each other: 6 is opposite 5. The reasoning: faces 2 and 3 meet along one edge, and that edge has two ends. At one end the third visible face is 5; at the other end it must be the face opposite to 5 — and Position II shows 6 there. Options 2 and 3 are ruled out immediately because they are seen together with 5 in Position I, and faces seen together are adjacent, never opposite.
  6. A cube of side 4 cm is painted on all six faces and then cut into 64 unit cubes of side 1 cm each. How many of these unit cubes have exactly two faces painted?

    1. 12
    2. 16
    3. 20
    4. 24
    Show answer

    Answer: D — 24

    Unit cubes with exactly two painted faces sit in the middle of an edge. A cube has 12 edges, and each edge holds (n − 2) middle cubes, so the count is 12(n − 2). For n = 4 this is 12 × 2 = 24. Check the whole cube: 8 corner cubes with 3 faces, 24 edge cubes with 2 faces, 6 × 2² = 24 face cubes with 1 face, and 2³ = 8 interior cubes with no paint — total 8 + 24 + 24 + 8 = 64 = 4³, which confirms the answer.
  7. A cube of side 5 cm is painted on all six faces and cut into 125 unit cubes of side 1 cm. Consider the following statements:
    1. Exactly 27 unit cubes have no face painted.
    2. Exactly 8 unit cubes have three faces painted.
    Which is/are correct?

    1. Only 1
    2. Only 2
    3. Both 1 and 2
    4. Neither 1 nor 2
    Show answer

    Answer: C — Both 1 and 2

    Both are correct. Unpainted cubes form the hidden interior, itself a cube of side (n − 2) = 3, so their number is 3³ = 27. Cubes with three painted faces are exactly the corner cubes, and a cube always has 8 corners, whatever n is — so 8. Full check for n = 5: 8 (three faces) + 12 × 3 = 36 (two faces) + 6 × 3² = 54 (one face) + 27 (none) = 125 = 5³.
  8. In triangle ABC, D is the midpoint of BC and E is the midpoint of AC. The two medians AD and BE are drawn and they meet at G. Counting every size, how many triangles are there in this figure?

    1. 5
    2. 6
    3. 7
    4. 8
    Show answer

    Answer: D — 8

    Count in groups. The two medians cut the triangle into four regions: three triangles ABG, BDG, AEG and one quadrilateral GDCE. Made of 1 region: ABG, BDG, AEG = 3. Made of 2 regions: ABD (ABG + BDG), ABE (ABG + AEG), ACD (AEG + GDCE), BCE (BDG + GDCE) = 4. Made of 4 regions: ABC = 1. No triangle uses exactly 3 regions. Total = 3 + 4 + 1 = 8, namely ABC, ABD, ABE, ABG, ACD, AEG, BCE, BDG. (With only ONE median drawn the count would be just 3.)