In this guide
In an odd-one-out question, most of the items share one property and one item does not. Your job is to name that shared property. Once you can say it in a few words ("all are prime", "all are fruits", "each letter jumps by two"), the odd item stands out.
This type is sometimes called classification, because you are sorting items into a class. It shows up regularly in the SSC GD reasoning section, in word, number and letter forms. It is also the reverse of analogy: in analogy you apply a relationship, and here you find the one item that breaks it. If you have practised analogy, you already know most of the groups.
How the question is asked
You will see one of three forms:
- Words: Rose, Lily, Lotus, Carrot. Which is different?
- Numbers: 11, 13, 17, 21, 23. Which does not belong?
- Letter groups or number pairs: BDF, GIK, MOQ, RTW. Or (2, 8), (3, 27), (4, 64), (5, 120).
The options are the items themselves. There is no trick in the wording. All the work is in finding the rule.
The method
- Look for the obvious group first. Fruits, cities, animals, instruments, planets.
- If more than one item looks odd, get more specific. "All are state capitals" is not enough if all four are capitals. Ask what else three of them share, such as having a sea port.
- For numbers, run the checklist below. Prime, square, cube, even or odd, multiples.
- For letters, write the positions (A = 1 … Z = 26) and look at the gaps.
- Check for a rival rule. Before you mark, ask whether another property picks out a different item. If it does, the question wants the rule that fits the others more exactly.
Word groups
| Group | Example | Odd one |
|---|---|---|
| Fruits | Apple, Mango, Banana, Potato | Potato (a vegetable) |
| Colours | Red, Blue, Green, Circle | Circle (a shape) |
| Planets | Mars, Venus, Moon, Jupiter | Moon (a satellite) |
| Water animals | Fish, Whale, Crab, Lion | Lion (lives on land) |
| Musical instruments | Flute, Tabla, Sitar, Hammer | Hammer (a tool) |
| Months | March, June, Monday, August | Monday (a day) |
| Units | Kilogram, Metre, Litre, Thermometer | Thermometer (an instrument) |
| Sense organs | Eye, Ear, Nose, Brain | Brain (not a sense organ) |
The number checklist
| Property | What to know | Example | Odd one |
|---|---|---|---|
| Prime | Primes up to 50: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47 | 3, 5, 7, 9 | 9 |
| Square | 1, 4, 9 … 400 (squares of 1 to 20) | 16, 36, 100, 40 | 40 |
| Cube | 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000 | 8, 27, 125, 150, 216 | 150 |
| Even and odd | Ends in 0, 2, 4, 6, 8 means even | 12, 18, 21, 24 | 21 |
| Multiples | Test 3, 5, 7, 9 and 11 | 27, 36, 45, 56, 63 | 56 (not a multiple of 9) |
Three traps sit inside this table. 1 is not prime. 2 is the only even prime. And some numbers look prime but are not: 51 = 3 × 17, 57 = 3 × 19, 91 = 7 × 13. Learn those three by heart.
Solved examples
Example 1 (words, more specific group). Kolkata, Chennai, Mumbai, Lucknow
- All four are state capitals, so that does not separate anything.
- Look deeper. Kolkata, Chennai and Mumbai are all major sea-port cities.
- Lucknow has no sea port. Answer: Lucknow.
Example 2 (numbers). 11, 13, 17, 21, 23
- All are odd, so even and odd does not help.
- Check primes: 11, 13, 17 and 23 are prime. 21 = 3 × 7.
- Answer: 21.
Example 3 (number pairs). (2, 8), (3, 27), (4, 64), (5, 120)
- Find the link inside each pair: 2³ = 8, 3³ = 27, 4³ = 64.
- For the last pair, 5³ = 125, not 120.
- Answer: (5, 120).
Example 4 (letter groups, equal gaps). BDF, GIK, MOQ, RTW
- Write positions: B D F = 2, 4, 6. G I K = 7, 9, 11. M O Q = 13, 15, 17.
- Each group jumps by +2, +2.
- R T W = 18, 20, 23. The gaps are +2, +3.
- Answer: RTW.
Example 5 (opposite letters). AZ, BY, CX, DV
- A (1) + Z (26) = 27. B (2) + Y (25) = 27. C (3) + X (24) = 27.
- D (4) + V (22) = 26. D's opposite is W, not V.
- Answer: DV.
Example 6 (square numbers). 25, 36, 49, 64, 80
- 25 = 5², 36 = 6², 49 = 7², 64 = 8².
- 80 is not a square. Check for a rival: even numbers are 36, 64 and 80, so even and odd does not pick out one item.
- Answer: 80.
Common mistakes
| Mistake | Correct way |
|---|---|
| Choosing the first item that looks odd | Name the rule for the other items, then check it |
| Stopping at a broad group | If all items fit the group, look for a narrower one |
| Treating 1 as prime, or 51 and 91 as prime | 1 is not prime; 51 = 3 × 17; 91 = 7 × 13 |
| Checking only the first gap in letter groups | Check every gap in every group |
| Missing a second property | Before marking, ask whether another rule picks a different item |
Practice set
- Rose, Lily, Lotus, Carrot
- Delhi, Mumbai, India, Chennai
- 4, 16, 36, 50
- 2, 3, 5, 8, 11
- BCD, FGH, JKL, MOP
- Hockey, Cricket, Football, Chess
- (3, 9), (4, 16), (6, 36), (7, 42)
- 27, 36, 45, 56, 63
Answers:
- Carrot. It is a vegetable; the others are flowers.
- India. It is a country; the others are cities.
- 50. 4, 16 and 36 are squares (2², 4², 6²). All four are even, so that is no rival.
- 8. The others are prime. 8 is also the only cube, so both rules agree.
- MOP. The others are three letters in a row; MOP skips N.
- Chess. It is a board game; the others are field sports played with a ball.
- (7, 42). In the others the second number is the square of the first. 7² = 49.
- 56. The others are multiples of 9 (3 × 9, 4 × 9, 5 × 9, 7 × 9).
What to do next
- Write the primes up to 50 and the squares up to 20² from memory.
- Solve 20 odd-one-out questions a day for a week, naming the rule aloud each time.
- For every question you get wrong, write the rule you missed in your mistake log.
- Move on to series, which uses the same number sense in order.
- Revisit the reasoning plan to see where this fits in your four weeks.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Staff Selection Commission website .
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