Number Analogy and Number Classification Tricks: Odd One Out Questions with Answers for SSC CGL, CHSL
Rahul Kumar
SSC Exam Expert & Content Editor
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In number analogy, a pair of numbers is related by some rule, and you apply the same rule to another number. In number classification, you find the odd number out of four or the odd pair out of four pairs. Both come in SSC CGL, CHSL and MTS reasoning papers. These number analogy and classification tricks for SSC list the common rules, a checking order and solved examples.
Common Relations Between Numbers
| Relation | Example |
|---|---|
| Square | 6 : 36 :: 8 : 64 |
| Cube | 3 : 27 :: 4 : 64 |
| Square root | 169 : 13 :: 225 : 15 |
| n times (n + 1) | 6 : 42 :: 8 : 72 |
| Multiply by a number | 5 : 15 :: 7 : 21 (multiply by 3) |
| Add a number | 9 : 12 :: 15 : 18 (add 3) |
| Sum of the digits | 27 : 9 :: 54 : 9 |
| Reverse of the number | 12 : 21 :: 35 : 53 |
Rules Used in Number Classification (Odd One Out)
| Check | Example of the odd number |
|---|---|
| Prime or not | 2, 3, 5, 9 → 9 is not prime |
| Perfect square | 16, 25, 36, 48 → 48 is not a square |
| Perfect cube | 8, 27, 64, 100 → 100 is not a cube |
| Multiple of a number | 24, 36, 48, 50 → 50 is not a multiple of 12 |
| Even or odd | 2, 4, 6, 9 → 9 is odd |
| Digit sum | 18, 27, 36, 41 → 41 (digit sum is 5, the others are 9) |
| Pair relation | (3, 9), (4, 16), (5, 25), (6, 30) → (6, 30), because the others are squares |
Checking Order
- Check whether the numbers are even or odd.
- Check prime, square and cube.
- Check the multiples of the same number (3, 4, 6, 7, 9, 11, 12).
- Check the sum of the digits.
- For pairs, find the relation of the first pair, and test it on the others.
Solved Examples
Example 1. 7 : 49 :: 9 : ?
The second number is the square of the first, so 9 × 9 = 81.
Example 2. 6 : 42 :: 8 : ?
6 × 7 = 42, so 8 × 9 = 72.
Example 3. Find the odd one out: 11, 13, 17, 21.
11, 13 and 17 are prime numbers, but 21 = 3 × 7. The odd one is 21.
Example 4. Find the odd one out: 27, 64, 125, 150.
27, 64 and 125 are cubes (3, 4 and 5 cubed). The odd one is 150.
Example 5. Find the odd one out: 14, 28, 42, 55.
14, 28 and 42 are multiples of 14. The odd one is 55.
Tips
- Learn the squares up to 25 and the cubes up to 10, because they appear most often.
- If three numbers share a feature (all prime, all even), the fourth is the odd one.
- In pairs, always write the relation of the first pair before you look at the options.
Quick Tips
- Learn the squares up to 25 and the cubes up to 10.
- Check prime, square, cube and multiples first.
- For pairs, find the relation in the first pair.
- Check the digit sum when nothing else works.
Quick Revision Points
- Number analogy applies the same rule on a new number.
- The odd number is the one that does not share the feature of the other three.
- Prime numbers up to 30: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29.
- n(n + 1) gives 2, 6, 12, 20, 30, 42, 56, 72.
- The digit sum test finds the odd one among multiples of 9.
Practice Questions With Answers
- 7 : 49 :: 9 : ?
Answer: 81 - 4 : 64 :: 5 : ?
Answer: 125 - 6 : 42 :: 8 : ?
Answer: 72 - 169 : 13 :: 225 : ?
Answer: 15 - Find the odd one out: 4, 9, 16, 20
Answer: 20 - Find the odd one out: 3, 5, 7, 9
Answer: 9 (the others are prime) - Find the odd one out: 27, 64, 125, 150
Answer: 150 - Find the odd one out: 14, 28, 42, 55
Answer: 55 - Find the odd one out: 19, 23, 29, 33
Answer: 33 - Find the odd one out: 12, 18, 24, 31
Answer: 31 (the others are multiples of 6)
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Reasoning notes page, read the previous topic Data Sufficiency Reasoning and the next topic Dice Reasoning. Also read Analogy and Classification.
Frequently Asked Questions
What is number analogy?
It is a question in which two numbers are related by a rule, and the same rule is applied to a third number.
How do I solve number analogy questions?
Find the rule of the first pair (square, cube, multiply, add) and apply it to the second pair.
What is number classification?
It is a question in which you find the number or the pair that does not belong with the other three.
What features should I check in odd one out?
Check prime, square, cube, multiples, even or odd and the sum of the digits.
Which prime numbers should I remember?
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.
What is n(n + 1)?
It gives the numbers 2, 6, 12, 20, 30, 42, 56 and 72 for n = 1 to 8.
Is 1 a prime number?
No. 1 is neither prime nor composite.
Is this topic asked in SSC exams?
Yes. Analogy and classification questions are asked in SSC reasoning papers.
How much time should I take?
About 20 to 30 seconds.
What are common mistakes?
Looking at only one feature, such as odd and even, and missing the square or the multiple.
How can I improve my speed?
Learn the squares and cubes tables and practise 10 questions daily.
What is the difference between analogy and classification?
In analogy you apply a rule to find a number, and in classification you find the number that breaks a rule.
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Data Sufficiency Reasoning: Statement I and II Questions with Tricks and Answers for SSC
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Dice Reasoning Tricks: Opposite Faces, Two and Three Positions of a Dice for SSC CGL, CHSL
Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.