Classification (Odd One Out) Notes

Last reviewed 22 Sept 2026 · 5 min read

The rule of the majority

Four items share one property and the fifth does not. The question is never "which do I dislike" but which property do three or four of them share. So:

Scan for the most obvious shared property (all are fruits, all are even, all are squares).
If everything fits, look for a second property (all are fruits that grow on trees — but the strawberry does not).
The odd one is whatever breaks the property that the majority keep.

Exam TipWhen two answers look right

Prefer the property that the largest number of items share, and the one that is sharper. Among rose, lily, lotus and marigold, "lotus grows in water" beats "lotus is my least favourite".

Words

Common bases for word classification:

Basis	Example set	Odd one
Category	Copper, Iron, Zinc, Brass	Brass (an alloy, not a metal)
Habitat	Whale, Shark, Dolphin, Seal	Shark (a fish; the rest are mammals)
Function	Pen, Pencil, Chalk, Scale	Scale (measures, does not write)
Origin	Cotton, Silk, Wool, Nylon	Nylon (synthetic)
Instrument family	Flute, Clarinet, Trumpet, Violin	Violin (string, the rest are wind)
Sphere	Ganga, Yamuna, Godavari, Nepal	Nepal (country, the rest rivers)
Whole vs part	Petal, Stem, Leaf, Plant	Plant (the whole)

Note the last row: when one option is the whole and the others its parts, the whole is the odd one — a favourite of exam setters.

Numbers

Test in this order — the answer is usually found within the first three:

Prime or composite (2, 3, 5, 7, 9 → 9 is composite)
Perfect square or cube (16, 25, 36, 48 → 48)
Divisibility (all divisible by 7 except one)
Digit rules: digit sum equal, palindrome, digits in order, both digits odd
Relation between the digits: second digit = first squared, or a fixed difference
Difference from the nearest square or cube: each is $n^{2} + 1$ except one

Letters and letter groups

Number the alphabet $A = 1$ to $Z = 26$ .

Fixed gaps: in ACE, BDF, CEG the letters rise by 2 each time; a group with a different gap is odd.
Reverse pairs: AZ, BY, CX all add to 27 — a pair that does not is odd.
Vowel position: in a group of three letters, most may have the vowel in the middle.
Same shift applied to a word: CAT → DBU is +1 on every letter; a group with an uneven shift is odd.

Worked examples

Worked ExampleExample 1 — a category break

Odd one out: Apple, Mango, Carrot, Banana.

Solution. Carrot — a vegetable (in fact a root) among fruits.

Worked ExampleExample 2 — the second property

Odd one out: Rose, Lily, Lotus, Sunflower.

Solution. All are flowers, so a second property is needed: Lotus grows in water, the rest on land.

Worked ExampleExample 3 — whole and parts

Odd one out: Engine, Wheel, Brake, Car.

Solution. Car — the others are its parts.

Worked ExampleExample 4 — numbers, primes

Odd one out: 17, 23, 27, 31.

Solution. 27 — the others are prime ( $27 = 3^{3}$ ).

Worked ExampleExample 5 — numbers, squares

Odd one out: 121, 144, 169, 180.

Solution. 180 — the others are perfect squares ( $1 1^{2}, 1 2^{2}, 1 3^{2}$ ).

Worked ExampleExample 6 — numbers, a digit rule

Odd one out: 24, 48, 84, 39.

Solution. 39 — each of the others is divisible by 4 (or: in the others the digits multiply to 8, 32, 32 while $3 \times 9 = 27$ ; the divisibility rule is the sharper one and is the intended answer).

Worked ExampleExample 7 — letters, gaps

Odd one out: ACE, BDF, CEG, DGI.

Solution. DGI — the first three rise by 2, 2; DGI rises by 3, 2.

Worked ExampleExample 8 — letters, opposite pairs

Odd one out: AZ, BY, CX, DV.

Solution. DV — the others are opposite pairs summing to 27; D(4) needs W(23), not V(22).

Worked ExampleExample 9 — the odd pair

Which pair is different? (a) Bat : Cricket (b) Racquet : Tennis (c) Puck : Hockey (d) Goal : Football

Solution. (d) — the first three are "equipment : the game it is used in"; a goal is an outcome, not equipment.

Worked ExampleExample 10 — a group of numbers

Odd one out: (4, 16), (5, 25), (6, 36), (7, 48).

Solution. (7, 48) — elsewhere the second number is the square of the first, and $7^{2} = 49$ .

Frequently tested points

Find the property shared by the majority, then name what breaks it.
If every item fits the first property, look for a sharper second one.
A whole among its parts, an alloy among metals, a synthetic among naturals and a fish among mammals are stock traps.
Numbers: check prime, square, cube and divisibility before anything clever.
Letters: gaps between positions, opposite pairs adding to 27, and the position of the vowel.
In "odd pair" questions the relationship inside each pair is what is being classified.

Common MistakeCommon mistakes

Picking an item for a reason only it satisfies ("the only one I have eaten") instead of a majority property.
Missing that all four fit the obvious property, so a second one is intended.
In letter groups, comparing the letters themselves instead of the gaps between their positions.
Rejecting the whole-and-part pattern because the whole "belongs" to the group.

Revision SummaryChapter summary

Classification asks which item breaks a property that the majority share.
Words are classified by category, function, origin, habitat and the whole-versus-part relation.
Numbers are tested for primality, squares and cubes, divisibility and digit rules, in that order.
Letter groups are compared by alphabet positions and the gaps between them, not by the letters themselves.
When two answers are defensible, the sharper and more widely shared property decides.

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Classification (Odd One Out)

The rule of the majority

Words

Numbers

Letters and letter groups

Worked examples

Frequently tested points

This chapter is in the syllabus of

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