Classification (Odd One Out)

A classification (odd-one-out) problem gives 4–5 items; four share a property, one doesn't. Your job: find the odd one.

Step 1 — Look for the common property among the majority. The property could be:

• All belong to the same category (e.g. all fruits, all metals).
• All have the same numerical property (all primes, all squares, all multiples of 3).
• Same letter pattern (all start with a vowel, all have 5 letters).
• Same word structure (all are verbs, all are colours).

Step 2 — The odd one violates the rule.

Common types:

1. Word groups: Lion, Tiger, Leopard, Cheetah, Wolf — first four are big cats, wolf is a canid.
2. Numbers — divisibility: 24, 32, 40, 48, 52 — first four are multiples of 8.
3. Numbers — squares/cubes: 4, 9, 16, 20, 25 — 20 is not a perfect square.
4. Letter groups by alphabet position: AB, EF, IJ, MN, QS — last is not consecutive letters.
5. Letter groups by vowel/consonant pattern: differ in vowels vs consonants count.
6. Number patterns: 36, 49, 81, 64, 48 — first four are squares.

Two-step rule — sometimes two properties combine: e.g. "primes greater than 10" — eliminate any composites and any below 11.

Trap: more than one item may seem odd. Always try to find a rule that makes exactly one item odd. If two are odd by one rule, the rule is wrong — try a different property.

Example 1: Wheat, Rice, Mustard, Barley.
Three are cereal crops. Mustard is the odd one (oilseed crop).

Example 2: 144, 196, 256, 270.
First three are perfect squares (12², 14², 16²). 270 is the odd one.

Example 3: AEIOU, BCDFG, JKMNP, RSTVW.
First is all vowels; rest are all consonants. AEIOU is the odd one.

Example 4: 27, 64, 125, 216, 350.
27 = 3³, 64 = 4³, 125 = 5³, 216 = 6³, 350 ≠ any cube. 350 is odd.

Example 5: Mercury, Venus, Mars, Moon.
First three are planets. Moon is a satellite.

Example 6: 15, 33, 45, 51, 57.
Check divisibility: all divisible by 3. Differ in some other property — let's check primes: 15 = 3×5 (not prime), 33 = 3×11, 45 = 3²×5, 51 = 3×17, 57 = 3×19. So all have 3 as a factor. Look more carefully: 15 → 1+5=6; 33 → 6; 45 → 9; 51 → 6; 57 → 12. So 45 and 57 have different digit sums from 15, 33, 51. Multiple anomalies — the question is poorly designed; in actual RRB, exactly one anomaly.

Example 7: Calcutta, Mumbai, Bangalore, Hyderabad, Lucknow.
First four are coastal/major-port-ish? No — only Mumbai and Calcutta are ports. Try: all 4 are state capitals of South+West Indian states; Lucknow is the only North Indian capital. Or: first four start with C/M/B/H (no common letter pattern). Best answer based on most-common RRB key: Bangalore isn't on the sea (but neither is Lucknow!). Modern best answer: Lucknow — North Indian city in a group of South/West cities.

Shortcut: in number questions, always check (in order): perfect squares, perfect cubes, primes, divisibility by small numbers (3, 4, 5, 7), digit-sum patterns.