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Classification (odd one out) for RRB Group D

Four or five items, all alike except one. Classification questions in RRB Group D use words, numbers, letter groups and number pairs. What to check, in what order, how to confirm you have the right property, with worked examples and practice.

17 Oct 2026 5 min read

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In this guide
  1. The "exactly one" test
  2. What to check, in order
  3. Worked examples
  4. Common mistakes
  5. Practice set
  6. What to do next

In a classification question, all the items share a property except one. Your job is to find the property, and the odd one appears. The syllabus calls this "classification" or "similarities and differences", and it comes in four forms: words, numbers, letter groups and number pairs.

These are quick questions, but they have a catch. Almost any group of items can be split in more than one way, so a candidate who grabs the first difference they notice can pick the wrong answer. The fix is a fixed checking order and one simple test, both explained below.

The "exactly one" test

A property is the right one only if it leaves exactly one item out. If it splits the items two and two, or three and two, it is not the rule. Keep checking.

If two different properties each leave out a different single item, choose the stronger, more specific property: a clear category (fruit, flower, organ) beats a surface detail (number of letters in the word). For numbers, a pattern that fits all the others (all squares, all multiples of 6) beats a detail that happens to pick out one number.

What to check, in order

Words

  1. Category: fruit, flower, organ, planet, unit, tool, city.
  2. Use or function: what the thing does or is used for.
  3. Place: where it is found, grows or lives.
  4. Measure: what a unit or instrument measures (length, mass, time).

Numbers

  1. Prime or not. Know the primes below 50: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.
  2. Perfect square or perfect cube.
  3. Even or odd.
  4. Divisible by the same number (3, 4, 5, 6, 7, 9, 11).
  5. Digit sum or a pattern in the digits.

Letter groups

  1. Gaps between letters: turn letters into positions (A = 1, … Z = 26) and check that each group uses the same steps.
  2. Opposite letters (pairs adding up to 27).
  3. Vowels and consonants, and where the vowel sits.

Number pairs

Check whether the second number comes from the first by one rule: square, cube, n² + 1, × a fixed number. The pair that breaks the rule is the answer.

Worked examples

Example 1: 13, 17, 19, 21, 23

  • 13, 17, 19 and 23 are prime. 21 = 3 × 7 is not.
  • Answer: 21. It is also the only multiple of 3, which agrees.

Example 2: Rose, Jasmine, Marigold, Mango

  • Rose, jasmine and marigold are flowers. Mango is a fruit.
  • Answer: Mango.

Example 3: 27, 125, 343, 225

  • 27 = 3³, 125 = 5³ and 343 = 7³ are perfect cubes. 225 = 15² is a square, not a cube.
  • Answer: 225.

Example 4: BDF, HJL, MOQ, RTW

  • BDF is 2, 4, 6; HJL is 8, 10, 12; MOQ is 13, 15, 17. Each group goes up by 2, then 2.
  • RTW is 18, 20, 23: up by 2, then 3.
  • Answer: RTW.

Example 5: Find the odd pair: 2–8, 3–27, 4–64, 5–120

  • 8 = 2³, 27 = 3³, 64 = 4³. The rule is "second number = first number cubed".
  • 5³ = 125, not 120. Answer: 5–120.

Example 6: Kilometre, Metre, Kilogram, Centimetre

  • Kilometre, metre and centimetre measure length. Kilogram measures mass.
  • Answer: Kilogram.

Common mistakes

  • Stopping at a property that splits the items two and two.
  • Missing a letter-gap change because the positions were counted on fingers instead of written down.
  • In number pairs, checking only the first pair and assuming the rule.
  • Using general knowledge that is uncertain. If you are not sure a city is a capital, check the other properties first.

Practice set

  1. 49, 121, 169, 143
  2. New Delhi, Paris, Tokyo, Mumbai
  3. 12, 18, 24, 29, 42
  4. ACE, GIK, MOQ, SUX
  5. Hammer, Saw, Chisel, Bucket
  6. Kidney, Liver, Heart, Femur
  7. DH, JN, PT, VY
  8. Find the odd pair: 4–16, 6–36, 7–49, 9–72

Answers:

  1. 143. 49 = 7², 121 = 11² and 169 = 13² are perfect squares. 143 = 11 × 13 is not.
  2. Mumbai. The others are national capitals (of India, France and Japan).
  3. 29. The others are multiples of 6. 29 is prime and odd.
  4. SUX. The others go up by 2 and 2 (1, 3, 5 · 7, 9, 11 · 13, 15, 17). SUX is 19, 21, 24: +2 then +3.
  5. Bucket. The others are tools for striking or cutting; a bucket is a container.
  6. Femur. It is a bone (the thigh bone). The others are organs.
  7. VY. DH, JN and PT each have a gap of 4 (4 → 8, 10 → 14, 16 → 20). VY is 22 → 25, a gap of 3.
  8. 9–72. The rule is "square": 4² = 16, 6² = 36, 7² = 49. But 9² = 81, not 72.

What to do next

  • Learn the primes below 50, the squares up to 30 and the cubes up to 15. Our squares and roots guide has the lists.
  • Solve 20 word and 20 number classification questions, writing the shared property for each answer.
  • For letter groups, write the positions above every letter until it becomes automatic.
  • Next, practise number and letter series, which use the same gap-checking skill.

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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