Odd One Out - Letters and Figures
~22 min read · AFCAT Reasoning and Aptitude
- Weight: about 1.5 marks per AFCAT paper (1 to 2 items), spread across letters, numbers, words and figures.
- Skill: find the single rule that THREE of the four items obey, then the fourth falls out automatically.
- Trap: picking the item that looks most different by eye, instead of testing it against a definite rule.
Overview
Odd One Out - Letters and Figures appears about 1.5 times per paper across the last four AFCAT solved papers, placing it in the high yield band of Reasoning and Aptitude.
Odd-one-out questions in AFCAT are short, neat rule-extraction tasks. You are given four items — letter clusters, words, numbers, pairs of words or small line figures — and asked which one does not belong. Three of them share a hidden rule. The fourth breaks that rule. Your job is to spot the rule first and the odd one second.
On a typical AFCAT Reasoning section of 25 to 30 questions, 1 to 2 belong to this topic. That is roughly 1.5 marks across the paper. The marks are easy if you have a checklist of rules to test, and they leak away if you guess by feel. This page builds that checklist for every flavour AFCAT has asked — letters, words, numbers, pairs and figures — gives you a worked-example tour of each, and shows you the trap patterns that catch most candidates.
The single most useful habit is this. Do not ask which one is odd. Ask which rule do three of them share. The moment you can name that shared rule in one short phrase, the odd one is forced.
Why odd-one-out is really a rule-extraction test
An odd-one-out item is not a comparison test. It is a rule-extraction test in disguise. The examiner has picked one rule — say perfect squares, or letters at positions 2 apart, or closed figures with one axis of symmetry — and built three items that obey it. The fourth is built to break it.
If you skim the four options and chase the one that looks unusual, you are doing the question backwards. You will sometimes get lucky, but on a tight item the wrong option will look more unusual than the right one, and you will lose the mark. The disciplined approach is:
- Look at the four items together.
- Find a short property that any three of them share.
- Check that the fourth does not share that property.
- Mark the fourth.
This is the same idea as a coding-decoding question or a series question — find the rule, apply the rule. Odd-one-out just exposes the rule by showing you three positive examples and one negative one.
The method — find the rule that three items share
Work through every odd-one-out item with the same four-step rhythm. With a little practice it takes about 20 seconds for an easy item and 50 seconds for a hard one.
- Classify the item type. Letter cluster, word, number, pair, or figure. Each type has its own short list of rules to test.
- Test the most common rule first. For letter clusters that is the alphabet-position gap. For numbers it is perfect squares or primes. For words it is broad category. For pairs it is the relationship between the two halves. For figures it is element count or symmetry.
- If three items satisfy the rule and one does not, you are done. Mark the one that breaks the rule.
- If two items break the rule, the rule is wrong. Drop it and try the next rule on the checklist. Do not force-fit.
Two warnings. First, the rule must be sharp. These numbers are small is not a rule. These numbers are perfect squares is a rule. Second, the rule must apply to exactly three items. If it applies to all four, it is too loose. If it applies to only two, it is too tight.
Letter-cluster odd-one-out — the rules table
A letter cluster is a short string of capital letters like BDF, KMO, PSV. The rule connecting three of them almost always lives in the alphabet positions of the letters — that is, A is 1, B is 2, C is 3 and so on up to Z which is 26.
Write the positions under the cluster before you do anything else. The rule will jump out.
| Rule | What to look at | Example of three matching, one not |
|---|---|---|
| Constant alphabet gap | Difference between consecutive letters in each cluster | BDF (gap 2), GIK (gap 2), MOQ (gap 2), PSV (gap 3, 3) — PSV is odd. |
| Mirror pair sum to 27 | First plus last letter position equals 27 | AZ (1+26=27), BY (2+25=27), CX (3+24=27), DV (4+22=26) — DV is odd. |
| Vowel-consonant pattern | Pattern of vowels (A E I O U) and consonants in the cluster | BAT (C-V-C), CAT (C-V-C), MAN (C-V-C), STR (C-C-C) — STR is odd. |
| All vowels or all consonants | Cluster entirely from one of the two groups | AEI, OUE, IUA (all vowels), AEK (has K) — AEK is odd. |
| Letter count | Number of letters in each cluster | ABC, DEF, GHI (3 letters), JKLM (4 letters) — JKLM is odd. |
| Alphabetical order | Whether the letters are in increasing alphabet order | ACE, BDF, MOQ (all rising), TRP (falling) — TRP is odd. |
| Repeated letter | Whether the same letter appears twice in the cluster | SEE, TOO, ADD (have a repeated letter), CAT (no repeat) — CAT is odd. |
| Sum-of-positions | Sum of the alphabet positions across the cluster | Three clusters sum to 30, one sums to 42 — the 42 one is odd. |
Run the gap test first. About 60 percent of AFCAT letter-cluster odd-one-out items resolve on the alphabet-position gap alone. If that fails, run mirror-sum, then vowel-consonant, then the rarer rules.
Word odd-one-out — by category and structure
When the four items are everyday words, the rule is almost always about meaning. Three words belong to one category and the fourth belongs to a related but different category. The temptation is to pick a word that sounds different — fight that.
| Rule type | What to look at | Example |
|---|---|---|
| Broad category | Three are members of one class; one is not | Apple, Mango, Banana, Potato — Potato is a vegetable. |
| Sub-category inside a broad class | All four are in the broad class but three share a sub-property | Apple, Mango, Banana, Pineapple — all fruit, but Banana, Mango, Pineapple are tropical, Apple is temperate. |
| Source or origin | Three come from plants, one from animals (or similar) | Milk, Butter, Cheese, Honey — Honey is from insects, others from cattle. |
| Function | Three serve the same function, one a different one | Hammer, Screwdriver, Saw, Pencil — Pencil is a writing tool, others are workshop tools. |
| Profession or actor | Three are people who do something, one is the object or place | Doctor, Engineer, Teacher, Hospital — Hospital is a place. |
| Animal classification | Mammal vs reptile vs bird vs fish | Cow, Goat, Dog, Crocodile — Crocodile is a reptile. |
| Number of syllables or vowels | Length structure of the word itself | Cat, Dog, Pen, Apple — Apple has two syllables, others have one. |
For word odd-one-out, test meaning rules in this order. Broad category, then sub-category, then source, then function. Only fall back to syllable count or vowel count if no meaning rule cleanly separates three from one. Syllable-based items are rare in AFCAT but do appear.
Number odd-one-out — the rules table
Number odd-one-out is the most rule-rich variant. The same four numbers can satisfy several rules at once, so you sometimes need to refine. The list below covers everything AFCAT has thrown.
| Rule | What to test | Example |
|---|---|---|
| Perfect square | Is the number n times n for some whole n? | 25, 36, 49, 50 — 50 is odd (others are 5², 6², 7²). |
| Perfect cube | Is the number n times n times n? | 27, 64, 125, 100 — 100 is odd (others are 3³, 4³, 5³). |
| Prime | Is the number divisible only by 1 and itself? | 11, 13, 17, 15 — 15 is odd (15 = 3 times 5). |
| Composite | Has divisors other than 1 and itself | 9, 21, 25, 23 — 23 is odd (it is prime). |
| Multiple of N | Divisible by a fixed small number | 12, 18, 24, 25 — 25 is odd (others are multiples of 6). |
| Even or odd | Parity of the number | 4, 6, 8, 7 — 7 is odd by parity. |
| Sum of digits | Add the digits; same sum for three | 12, 21, 30, 41 — 41 sums to 5, others sum to 3. |
| Palindromic number | Reads the same forwards and backwards | 121, 343, 565, 124 — 124 is odd. |
| Fibonacci | Each term is the sum of the previous two: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89... | 13, 21, 34, 30 — 30 is odd. |
| Factorial | n! values: 1, 2, 6, 24, 120, 720... | 6, 24, 120, 100 — 100 is odd. |
| Triangular numbers | 1, 3, 6, 10, 15, 21, 28, 36... | 15, 21, 28, 30 — 30 is odd. |
Run the checklist in this order. Perfect squares first, then cubes, then primes, then multiples of small numbers, then digit sums, then the rarer ones. Most AFCAT number items resolve on squares, primes or multiples.
Pair odd-one-out — the relationship has to match
A pair odd-one-out gives you four small pairs of words, like (Cat : Kitten), and asks which pair does not fit. The trick is that the rule lives in the relationship between the two halves, not in the two words on their own.
Classic example. Cat : Kitten, Dog : Puppy, Cow : Calf, Horse : Mare. The first three are adult to young. Horse : Mare is male adult to female adult. Horse : Mare is odd. The young of a horse is a foal.
Other relationship families that appear in AFCAT:
- Worker to tool. Carpenter : Saw, Tailor : Scissors, Cobbler : Awl, Doctor : Stethoscope.
- Worker to workplace. Doctor : Hospital, Teacher : School, Judge : Court, Engineer : Bridge — Engineer : Bridge breaks the pattern; the place is Site or Office.
- Animal to home. Bee : Hive, Lion : Den, Bird : Nest, Cow : Stable.
- Country to capital. India : Delhi, France : Paris, Japan : Tokyo, China : Shanghai — Shanghai is not the capital, Beijing is.
- Instrument to player. Flute : Flautist, Violin : Violinist, Piano : Pianist, Guitar : Strings — last is item to part, not item to player.
- Synonym pairs. Big : Large, Tiny : Small, Quick : Fast, Happy : Sad — the last is antonyms.
- Cause and effect. Rain : Flood, Spark : Fire, Wind : Erosion, Cloud : Sky — the last is part to whole.
Name the relationship in one short phrase. Adult to young, worker to tool, country to capital. If three pairs match that phrase and one does not, you are done. If the relationship is fuzzy, sharpen it. Worker to tool is fine. Worker to thing is too loose — almost anything satisfies it.
Figure odd-one-out — what to count and compare
Figure odd-one-out gives you four small line drawings. Three share a geometric property, the fourth does not. Counting beats eyeballing — count something specific before you compare shapes.
| Rule | What to count or check | Example of three vs one |
|---|---|---|
| Element count | Number of sides, dots, line segments, arrows, circles | Three figures contain 4 dots, one contains 5 dots — the 5-dot one is odd. |
| Closed vs open | Whether the boundary forms a closed shape | Triangle, square, hexagon, L-shape — L-shape is open or non-polygon. |
| Straight vs curved | Sides are all straight or include curves | Triangle, square, pentagon, circle — circle is the only curved one. |
| Symmetry axes | Number of axes of symmetry | Three figures have one vertical axis of symmetry, one has none. |
| Rotation vs reflection | Whether figures map to each other by rotation only or need flipping | Three figures are 90-degree rotations of each other; one is a mirror image and needs flipping. |
| Inner element type | What sits inside the boundary — dot, cross, arrow | Three figures have a dot inside, one has a cross inside. |
| Direction of arrow | Where an arrow inside the figure points | Three arrows point right or up, one points left or down. |
| Shading or fill | Whether interior is shaded, half-shaded or empty | Three are half-shaded, one is fully shaded. |
Before you guess, count three things in each figure — number of sides, number of dots, number of inner marks. Most AFCAT figure odd-one-out items resolve on a count or a closed-vs-open check. Symmetry tests come up in the harder items.
Two-rule items — when one option breaks two rules
Some odd-one-out items are sharper than they look. Two options might both fail one or another simple rule, and you have to decide which is more odd. The right rule is the one that exactly three items satisfy.
Suppose the four numbers are 16, 25, 36, 49 — all perfect squares. Add a fifth scenario: 16, 25, 36, 50. Here 50 fails the perfect-square rule. The rule is clean: three are squares, one is not. Mark 50.
Now suppose 16, 25, 36, 27. Both 27 and the rest behave differently — 27 is a perfect cube while 16, 25, 36 are perfect squares. The rule is still perfect square: 16, 25, 36 are perfect squares, 27 is not. Mark 27. Do not be distracted by the fact that 27 happens to have its own pretty property.
The lesson. The shared rule has to fit exactly three items. If you find a rule that fits three, the fourth is the answer — even if that fourth item has its own property that distracts the eye. The odd item is not the one with a striking property; it is the one missing the shared property.
Refine-the-rule tactic when more than one option could be odd
Sometimes your first rule fits two items and breaks two. That means the rule is wrong, not that the question is broken. Refine.
Example. Apple, Mango, Banana, Tomato. First rule: fruits. Apple, Mango, Banana are fruits; Tomato is botanically a fruit too. So the rule does not separate cleanly. Refine to sweet fruits commonly eaten as fruit. Apple, Mango, Banana fit; Tomato does not. Mark Tomato.
Example. 4, 9, 16, 25. First rule: even. Only 4 and 16 are even. The rule is wrong. Refine to perfect square. All four are squares — the rule is too loose. Refine again to perfect square of a prime. 4 = 2², 9 = 3², 25 = 5², all primes; 16 = 4², 4 is not prime. Mark 16.
The pattern. Move from a loose property to a tighter one until exactly three items satisfy it. If you end up at exactly three items, you have found the rule. If you cannot get below four or above two, your first guess was on the wrong axis — try a different family of rule (digit-based instead of value-based, structural instead of categorical).
Common AFCAT trap patterns
The traps in odd-one-out are predictable. Knowing them saves a mark per paper across the year.
- Pick by appearance, not by rule. Three letter clusters look ordinary; the fourth has a long or unusual shape. The unusual-looking one is sometimes the odd one and sometimes not. You must check.
- Multiple rules at once. Three numbers might all be perfect squares and all be even. The fourth might break only one of those. Decide which rule the examiner is using by checking which separates three from one cleanly.
- Categorical overlap. Tomato is both fruit and vegetable; whale is both mammal and sea creature; English is both subject and language. The examiner is testing whether you know the cleaner category.
- Hidden alphabet. A word odd-one-out can occasionally hinge on the alphabet structure, not on meaning. Cat, Dog, Pen, Apple — Apple is the only word with a repeated letter, or the only two-syllable word. Test alphabet structure if meaning fails.
- Hidden numeric property in letters. A letter cluster can be odd because the sum of its letter positions differs. If the gap rule and the vowel rule both fail, sum the positions.
- Figure-count trap. Three figures have 5 line segments each, one has 6. The line count is the rule; do not be distracted by the overall shape looking similar.
- Mare trap on pair items. Adult-to-female-adult slipped in among adult-to-young pairs. Always name the relationship in full and test all four pairs against it.
- Capital-city trap. Shanghai for Beijing, Sydney for Canberra, Karachi for Islamabad, Istanbul for Ankara. Know the actual capital, not the most famous city.
Time budget and approach in the exam
AFCAT gives you 120 minutes for 100 questions — 72 seconds per question on average. Reasoning is faster than General Awareness for most candidates, so a smart split gives you 50 to 60 seconds for an odd-one-out item.
| Item type | Target time | Notes |
|---|---|---|
| Letter cluster — gap rule | 20 to 30 seconds | Should be near-automatic once you write positions. |
| Letter cluster — mirror or vowel rule | 30 to 45 seconds | Write positions, test mirror sum, test vowel pattern. |
| Number — square or prime | 20 to 30 seconds | Squares up to 400 and primes up to 100 should be memorised. |
| Number — digit sum or refined rule | 40 to 60 seconds | Quick mental check; do not let it cross a minute. |
| Word — broad category | 15 to 25 seconds | Usually visible at a glance once you name the category. |
| Pair — relationship test | 30 to 45 seconds | Name the relationship in one phrase, then check all four. |
| Figure — count and compare | 40 to 55 seconds | Count three properties, not just one. |
If an item is still open after 75 seconds, mark your best guess (you have eliminated at least one option by then), flag it for review and move on. AFCAT has a negative-marking penalty of one mark per wrong answer, so do not blind-guess — but an answer narrowed to two options is worth attempting on expected value, since +3 with 50 percent confidence beats skipping.
Worked AFCAT-style examples
Find the odd one out: BDF, GIK, MOQ, PSV.
Write positions. BDF = 2, 4, 6 (gap 2). GIK = 7, 9, 11 (gap 2). MOQ = 13, 15, 17 (gap 2). PSV = 16, 19, 22 (gap 3). The shared rule is constant gap of 2 between consecutive letters. PSV breaks it.
Find the odd one out: AZ, BY, CX, DV.
Mirror sum rule. AZ = 1 + 26 = 27. BY = 2 + 25 = 27. CX = 3 + 24 = 27. DV = 4 + 22 = 26. The first three pairs are mirror pairs about the centre of the alphabet (D should pair with W, not V). DV breaks the rule.
Find the odd one out: 25, 36, 49, 50.
Check perfect squares. 25 = 5 times 5, 36 = 6 times 6, 49 = 7 times 7. 50 is not a square. Rule: perfect squares. 50 breaks it.
Find the odd one out: 11, 13, 17, 15.
Check primes. 11, 13, 17 are prime (only divisible by 1 and themselves). 15 = 3 times 5, so it has divisors. Rule: prime numbers. 15 is the odd one.
Find the odd one out: 8, 21, 34, 40.
Check Fibonacci. The Fibonacci numbers are 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89... 8, 21, 34 all sit in the Fibonacci sequence. 40 does not. Rule: Fibonacci numbers. 40 breaks it.
Find the odd one out: Apple, Mango, Banana, Carrot.
Broad category test. Apple, Mango, Banana are fruits. Carrot is a vegetable (a root vegetable). Rule: fruits. Carrot breaks it.
Find the odd one out: Doctor, Engineer, Teacher, Hospital.
Doctor, Engineer and Teacher are professions — people who do a job. Hospital is a place, specifically where a doctor works. Rule: profession. Hospital breaks it.
Find the odd one out: Cat-Kitten, Dog-Puppy, Cow-Calf, Horse-Mare.
Relationship test. Cat-Kitten, Dog-Puppy and Cow-Calf are all adult-to-young pairs. Horse-Mare is male-adult to female-adult — Mare is the female horse, not the young. The young of a horse is a foal. Rule: adult to young of the same species. Horse-Mare breaks it.
Find the odd one out: India-Delhi, France-Paris, Japan-Tokyo, China-Shanghai.
Relationship is country-to-capital. Delhi is the capital of India, Paris of France, Tokyo of Japan. Shanghai is not the capital of China; Beijing is. Shanghai is the largest city but not the capital. China-Shanghai breaks the rule.
Find the odd one out: triangle, square, pentagon, circle.
Triangle, square and pentagon are polygons — closed figures made of straight sides. Circle is closed but has no straight sides. Rule: straight-sided closed figures. Circle breaks it.
Find the odd one out: 121, 343, 565, 124.
Palindrome test. 121 reads 121 backwards. 343 reads 343. 565 reads 565. 124 reads 421. Rule: palindromic three-digit numbers. 124 breaks it.
Find the odd one out (figures): three figures contain four dots inside a square; the fourth figure contains five dots inside a square.
All four figures share the same boundary (a square). Three of them contain exactly four dots; one contains five. Count the elements before judging by shape. Rule: square boundary with four interior dots. The five-dot figure breaks it.
Find the odd one out: 27, 64, 125, 100.
Cube test. 27 = 3 cubed, 64 = 4 cubed, 125 = 5 cubed. 100 = 10 times 10 (a square, not a cube). Rule: perfect cubes. 100 is the odd one. (Note: 100 is itself a perfect square, but the shared rule among three is cubes, so 100 is out.)
Exam-day strategy
- Name the rule before you name the odd one. Say it in one short phrase such as 'perfect squares' or 'adult to young'.
- Classify the item type first — letters, words, numbers, pairs or figures — then walk the rule checklist for that type.
- For letter clusters, always write the alphabet positions under the letters before testing rules.
- For numbers, run perfect squares, perfect cubes, primes and small-multiples in that order. They resolve most items.
- For words, test broad category first, then sub-category, then source or function. Only drop to syllable count if all meaning rules fail.
- For pair items, write the relationship between the two halves in one short phrase, then test all four pairs against that phrase.
- For figure items, count something specific — sides, dots, inner marks — before comparing overall shape.
- If a rule fits all four items, it is too loose; if it fits only two, it is too tight. The correct rule fits exactly three.
- When two options could both be odd, pick the more specific shared rule that separates three from one cleanly.
- Cap yourself at 90 seconds per item. If still unsure, mark the best of the remaining two and move on; do not blind-guess from four options.
Practise Odd One Out - Letters and Figures for AFCAT
Practise AFCAT-style odd-one-out items across letters, numbers, words, pairs and figures, with full rule-extraction explanations.
Start free AFCAT practiceFrequently asked questions
How many odd-one-out items does AFCAT have per paper?
About 1 to 2 per paper, averaging 1.5 marks. They sit inside the Reasoning section, which carries 25 to 30 questions overall.
What kinds of odd-one-out items does AFCAT ask?
Five broad types — letter clusters, words, numbers, word pairs and small figures. Letter clusters and numbers are the most common; figures and pairs appear in roughly one paper out of two.
What is the most common letter-cluster rule?
Constant alphabet gap between consecutive letters. About 60 percent of letter-cluster items in AFCAT resolve on this rule. Always write positions first and check the gap before testing anything else.
Which number rules show up most often?
Perfect squares, perfect cubes, prime numbers and multiples of small numbers (2, 3, 5, 6, 9). Memorise squares up to 400 and primes up to 100 so the checks are instant.
What if two options both seem odd?
Sharpen the rule. Move from a loose property (large numbers, contains vowels) to a tighter one (perfect square of a prime, vowels in alphabetical order) until exactly three items satisfy it. The one that does not is your answer.
Should I guess if I cannot find the rule?
Eliminate first. Even one minute of work on an odd-one-out item usually cuts the four options down to two. Guessing between two options carries roughly 50-50 expected value at +3 / -1, which is positive. Do not blind-guess from a full set of four; the expected value is then -0.5 if you have no signal.
How fast should I be on this topic?
Aim for 40 to 55 seconds on a typical odd-one-out item, and cap any single item at 90 seconds. The marks per minute fall sharply after the first minute, so quick moves on are usually worth it.