In this guide
In a classification question, three items belong together and one does not. The exam phrases it as "select the odd one out" or "three of the following are alike in some way; find the one that is different". The items can be words, numbers, number pairs or letter groups.
The skill is the same as in analogies: find the property the three share. The extra step is making sure no other property picks out a different item. Most wrong answers in this topic come from stopping at the first property that looks right.
A method that works on every type
- List the obvious property of each item (category, prime or not, gap between letters).
- Look for a 3–1 split. The property you want groups three items and leaves one out.
- Check one rival property. Is there a second property that splits the items 3–1 in a different way? If yes, look for a stronger, more specific property.
- Choose the item that breaks the most specific shared property.
A property that splits the items 2–2 is not the intended one. Drop it and move on.
What to check, by type
Words
| Check | Example group | Odd one |
|---|---|---|
| Category | Apple, Mango, Potato, Banana | Potato (a vegetable) |
| Function or use | Saw, Hammer, Chisel, Stethoscope | Stethoscope (a doctor's instrument) |
| Where it moves | Car, Bus, Truck, Boat | Boat (travels on water) |
| Unit and quantity | Metre, Kilometre, Centimetre, Kilogram | Kilogram (measures mass) |
| Shape or dimension | Circle, Rectangle, Rhombus, Sphere | Sphere (a 3D solid) |
Numbers
Check these in order. They are also the usual rival properties, so run through all of them before you answer.
- Prime or composite. Remember that 1 is not prime and 2 is the only even prime.
- Perfect square or cube.
- Odd or even.
- Divisible by a common number (3, 5, 7, 11).
- Digit sum or a pattern in the digits.
Number pairs
Check the rule inside each pair: square (4, 16), cube (3, 27), multiple (5, 35), or n² ± 1 (6, 37).
Letter groups
- Gaps between the letters (+ 2, + 2 or + 1, + 2, + 3).
- Opposite pairs (positions add up to 27).
- Vowels and consonants.
- Position sums (rarely asked, but useful as a last check).
Worked questions
Q1. Find the odd one out: 3, 5, 9, 11
3, 5 and 11 are prime; 9 = 3 × 3 is not. Rival check: all four are odd, so odd–even does not split them. Answer: 9.
Q2. Find the odd one out: 27, 64, 125, 150
27 = 3³, 64 = 4³ and 125 = 5³ are cubes; 150 is not. Rival check: two are odd (27, 125) and two are even (64, 150), a 2–2 split, so it is not the intended property. Two are divisible by 5 (125, 150), also 2–2. Answer: 150.
Q3. Find the odd one out: Rose, Marigold, Jasmine, Banyan
Rose, marigold and jasmine are flowers. The banyan is a tree. Answer: Banyan.
Q4. Find the odd one out: ACE, BDF, GIK, MOR
ACE: + 2, + 2. BDF: + 2, + 2. GIK: + 2, + 2.
MOR: M → O is + 2, but O → R is + 3. Answer: MOR.
Q5. Find the odd one out: (3, 9), (4, 16), (5, 25), (6, 30)
In the first three pairs, the second number is the square of the first. In (6, 30), 30 is 6 × 5, not 6². Answer: (6, 30).
Q6. Find the odd one out: AZ, BY, CX, DV
A + Z = 1 + 26 = 27. B + Y = 2 + 25 = 27. C + X = 3 + 24 = 27. D + V = 4 + 22 = 26. The first three are opposite pairs. Answer: DV.
A question with a hidden rival
Suppose the options are 4, 9, 16, 20. Most candidates see squares (4, 9, 16) and choose 20. But 9 is the only odd number, which is also a 3–1 split. Strictly, a group like this has two defensible answers.
A well-set question avoids this, and in the exam the options usually settle it. When both properties seem to work, prefer the more specific one: "perfect square" says more about a number than "odd". Treat such a question as a warning to slow down, not a reason to panic.
Common mistakes
- Stopping at the first property without checking all four items.
- Treating 1 as prime, or forgetting that 2 is prime.
- Calling 91 prime. It is 7 × 13. Also watch 51 (3 × 17) and 87 (3 × 29).
- Missing a change in the middle of a letter group, as in MOR.
- Using general knowledge that is too clever. The intended property is usually the simple one a Class 10 student would see.
Practice set
- Wheat, Rice, Maize, Mustard
- 25, 49, 91, 121
- 13, 17, 21, 23
- Ganga, Godavari, Krishna, Nile
- BCD, FGH, JKL, MNP
- DF, HJ, LN, PS
- (2, 8), (3, 27), (4, 64), (5, 120)
- Triangle, Square, Pentagon, Cube
Answers:
- Mustard. Wheat, rice and maize are cereals; mustard is an oilseed.
- 91. 25, 49 and 121 are squares (5², 7², 11²). 91 is 7 × 13, so it is neither a square nor a prime.
- 21. 13, 17 and 23 are prime; 21 = 3 × 7.
- Nile. The other three are Indian rivers that flow into the Bay of Bengal.
- MNP. BCD, FGH and JKL are consecutive letters; MNP skips O.
- PS. DF, HJ and LN have a gap of 2; P → S is a gap of 3.
- (5, 120). In the others, the second number is the cube of the first; 5³ is 125.
- Cube. It is a 3D solid; the others are flat shapes.
What to do next
- Memorise primes up to 100 and squares up to 20². Half of number classification is recognising them instantly.
- For your next 20 questions, write the rival property you checked next to each answer.
- Revise analogies, which use the same relationship-spotting skill.
- Then move to number series.
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 Railway Recruitment Boards website .
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