Classification (Odd One Out)
🔒 Log in to trackClassification questions list four or five similar items and ask which one does not belong. The hidden rule — a shared category, number property or letter structure — must hold for every option except one. CGL asks 2-4 per shift, drawn from number properties, letter clusters, word categories and pairs.
One page per subtopic: detailed notes, every question type, formulas, tricks and practice sets.
Every formula on one printable page, grouped by subtopic.
4 exam-level questions worked step by step.
89 questions — untimed practice or a timed test with analysis.
Track record in the exam
Questions per shift in recent SSC CGL papers.
Test difficulty mix (89 questions)
Question patterns exams keep repeating
Taken from previous-year papers. If a pattern is marked "very common", expect to see it in your exam.
Standard odd one out (three alike, one different)
very commonFour items - numbers, letters, words or mixed - where exactly three share one clear feature.
How to solve: Test objective rules first (spelling, letter steps, number properties), then function, then category. The rule must keep exactly three items together; the item that breaks it is the answer.
Example: 17, 26, 35, 44
All end in 5 or 6 (trap). Real rule: +9 each. That fits all - so go deeper: digit sums are 8, 8, 8, 8 too. Every option obeys +9, so the tie-break is another property: 17 is prime, the rest are not → 17.
Two-level (tie-break) odd one out
commonThe first rule you find keeps all four (or none) together, so a second, deeper property decides.
How to solve: When level 1 fails (all squares, all even, all +7), test the next layer: are the roots prime? Is the digit sum odd? Keep going until exactly one item splits away.
Example: 121, 169, 225, 289
All are perfect squares (11², 13², 15², 17²). Deeper: 11, 13, 17 are prime but 15 is not → 225.
Category break (animals, plants, living things)
very commonFour words from nearby nature groups: animals, birds, fruits, vegetables, flowers, trees.
How to solve: Name the group of three. The traps are words from the same habitat or colour - the answer is the one from a different biological category (mammal among reptiles, vegetable among fruits).
Example: Snake, Lizard, Crocodile, Whale
All live in/near water, but snake, lizard and crocodile are reptiles; the whale is a mammal → Whale.
Function / use break
commonFour objects or devices, three of which serve the same function.
How to solve: Ask 'what does it do?' rather than 'where is it found?'. Three input devices and one output device, or three measuring instruments and one storage item - the different function is the answer.
Example: Keyboard, Mouse, Scanner, Monitor
Keyboard, mouse and scanner send data into the computer (input devices); the monitor only displays it (output) → Monitor.
Prime vs composite
very commonFour numbers of similar size; three are prime or three are composite.
How to solve: Test divisibility by 2, 3, 5, 7 quickly. Watch traps: 91 = 7 × 13 and 87 = 3 × 29 look prime but are not; 2 is the only even prime.
Example: 17, 19, 23, 27
17, 19, 23 are prime. 27 = 3 × 9 (or 3³) is composite → 27.
Perfect square / perfect cube
commonThree numbers are squares (or cubes) and one is not, though all look 'special'.
How to solve: Know squares to 20² = 400 and cubes to 12³ = 1728. Then apply the tie-break if needed: of the squares, are three roots prime?
Example: 64, 125, 216, 100
64 = 4³, 125 = 5³, 216 = 6³ are cubes; 100 is only a square → 100.
Constant step inside the cluster
commonFour letter clusters; in three of them the letters move by the same step every time.
How to solve: Write the step tuple for each cluster (positions via EJOTY). Three clusters share one tuple (e.g. +2, +2); the odd one has a different tuple or unequal steps.
Example: BDF, HJL, NPR, PQR
B→D→F, H→J→L, N→P→R all jump +2, +2; P→Q→R jumps +1, +1 → PQR.
Word pairs: relation break
very commonFour pairs; three share the same relation (adult : young, worker : tool, male : female).
How to solve: Say the relation of the first pair as a sentence, then test each option with the same sentence. The pair with a different relation (or a reversed direction) is the odd one.
Example: Cow : Calf, Horse : Foal, Dog : Kitten, Sheep : Lamb
Calf, foal and lamb are the correct young ones; a kitten is the young of a cat, not a dog → Dog : Kitten.
GK pairs: one factually wrong pair
commonFour country-capital, country-currency or instrument-quantity pairs; one pair states a wrong fact.
How to solve: Check each fact, not the relation's shape - all four have the same shape, but one pairing is factually wrong.
Example: Rome : Italy, Oslo : Norway, Madrid : Portugal, Cairo : Egypt
Madrid is the capital of Spain, not Portugal → Madrid : Portugal is the wrong pair.
Chain sets (a, b, c): same operation twice
commonSets of three numbers such as (2, 4, 8), (3, 6, 12); three sets follow one rule, one set breaks it.
How to solve: Find the repeated step of each set (×2, ×2 or +5, +5). The set whose middle step or last number breaks the chain is the odd one.
Example: (2, 4, 8), (3, 6, 12), (5, 10, 20), (4, 9, 16)
The first three double each step (×2, ×2). (4, 9, 16) is a squares chain (2², 3², 4²) → (4, 9, 16).