60-second revision
- Classification = three items share a property, one does not. Find the shared property first, then the outsider.
- Words are usually grouped by category, function, habitat, material or place type.
- Numbers are grouped by prime/composite, squares/cubes, even/odd, divisibility or digit patterns.
- Primes up to 50: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47. 1 is not prime; 2 is the only even prime.
- Number pairs follow one rule (n², n³, n × k); test the rule on every pair.
- Letter groups: write positions and compare the gaps; also check vowels and opposite letters.
- If two items look odd, the property that unites the remaining three most exactly is the intended one.
How classification works
In a classification question four (sometimes five) items are given and all but one share a property. You must name that property and pick the outsider. RRB calls the same skill "similarities and differences" and sometimes flips it: three items are given and you pick the option that belongs with them. The method is identical — find the shared property.
- Look for a property that exactly three items share.
- If more than one item seems odd, look for a narrower property (planets versus inner planets, bones versus bones of the arm).
- Prefer a clear, factual property over a far-fetched one (spelling length or first letters are used only when nothing else fits).
Word classification — common bases
| Basis | Group of three | Odd one |
|---|---|---|
| Category | Rose, Lotus, Lily | Banyan (a tree) |
| Function / use | Pen, Pencil, Chalk (write) | Eraser |
| Habitat | Whale, Shark, Octopus (sea) | Camel |
| Type of instrument | Tabla, Mridangam, Dholak (percussion) | Shehnai (wind) |
| Place type | Jaipur, Chennai, Shimla (state capitals) | Jodhpur |
| Unit type | Metre, Mile, Furlong (length) | Litre (volume) |
| Body part type | Ulna, Humerus, Patella (bones) | Triceps (muscle) |
Number classification
Run through these checks in order: prime or composite; perfect square or cube; even or odd; divisible by 3, 7, 11 or 13; and digit patterns (sum of digits, middle digit = sum or difference of outer digits, digits in increasing order).
| Property | Group of three | Odd one |
|---|---|---|
| Prime numbers | 31, 37, 41 | 39 (3 × 13) |
| Perfect squares | 144, 196, 256 | 200 |
| Perfect cubes | 27, 125, 343 | 210 |
| Divisible by 7 | 91, 119, 133 | 143 (11 × 13) |
| Digit sum 12 | 345, 516, 822 | 434 (sum 11) |
| Middle digit = difference of outer digits | 725, 936, 844 | 623 |
Worked check for the last row: 725 gives 7 − 5 = 2 (middle 2), 936 gives 9 − 6 = 3 (middle 3), 844 gives 8 − 4 = 4 (middle 4), but 623 gives 6 − 3 = 3 while its middle digit is 2. Always verify every item, not just the first two, because a well-set question has exactly one outsider.
Number pairs and triplets
For pairs, find one rule that turns the first number into the second (square, cube, × 3 + 1, n × (n + 1)) and test it on every pair. Example: 4 : 20, 5 : 30, 6 : 42, 7 : 54 — the rule n × (n + 1) gives 20, 30, 42 and 56, so 7 : 54 is odd.
Letter-group classification
Convert letters to positions (EJOTY anchors: E 5, J 10, O 15, T 20, Y 25). Then compare: the gaps between letters, whether the group contains a vowel, whether the letters are opposites (sum 27), or whether the group moves forward or backward.
| Group | Positions | Pattern |
|---|---|---|
| CFI | 3, 6, 9 | +3, +3 |
| KNQ | 11, 14, 17 | +3, +3 |
| PSW | 16, 19, 23 | +3, +4 — odd one |
| AZ, BY, CX | sum 27 | opposite pairs |
Routine in the exam
- Spend the first 10 seconds finding a category; if none, switch to numbers or positions.
- Verify the property on all three "normal" items before marking.
- If you find a property shared by only two items, keep looking — it is not the intended basis.
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