What is asked
"Four of the following five are alike in a certain way and so form a group. Which one does not belong?" The items can be letter groups, number groups, number pairs or words (category based). Usually 0-1 question in Prelims, but it is a quick mark.
Letter-group checks (in this order)
- Gaps between letters: BDF (+2, +2) vs CEH (+2, +3).
- Opposite pairs: A-Z, C-X, G-T (positions add to 27).
- Vowel / consonant pattern, or position of vowels.
- Letters in alphabetical order inside a word (BEGIN, FIRST, CHINTZ) vs not (WORLD).
Number checks
- Prime / composite: 17, 23, 29, 41 vs 33.
- Squares and cubes: 121, 169, 225, 289 are squares of odd numbers; 324 = 18 squared is the odd one.
- Pairs: second = first cubed (3 : 27), second = first squared + 1 (4 : 17).
- Digit patterns: 246, 357, 468 (each digit +2) vs 689.
- Divisibility, sum of digits, odd/even.
Word categories
Planets vs satellites (Moon), the cat family vs the dog family (wolf), payment systems vs economic indicators (GDP among NEFT, RTGS, IMPS, UPI), flowers vs fruits. Choose the most specific common property - "all are animals" is too broad if four are big cats.
Method
- Check the type: if all items are words, think of categories first; if all are numbers or letter groups, look for a calculation or a gap pattern.
- Test one simple property on all five items (odd/even, prime, square, +2 gaps). Stop as soon as exactly four agree.
- Confirm the odd item fails the rule for a clear reason, and that no second item also fails.
Worked example
3 : 27, 4 : 64, 5 : 125, 6 : 216, 7 : 334. The first four follow "second number = cube of the first". 7 cubed is 343, so 7 : 334 is the odd one.
Trap: if two different rules each make a different option odd, look for the rule under which exactly four options agree. A rule that leaves two options odd is the wrong rule.
More traps
- Do not stop at the first odd property you notice. In 121, 169, 225, 324, 289, "only 324 is even" and "only 324 is the square of an even number" point to the same answer, which confirms it.
- For word groups, avoid rules based on spelling (number of letters) unless no meaning-based rule works - setters usually intend a category.
Time target
20-30 seconds. If no rule appears in 40 seconds, skip.