The odd number — Aptitude Question
The odd number: Which one does not belong? 3, 5, 7, 9, 11 Answer and worked solution on CodeKairo.
- Topic: Analogies & Classification
- Section: Logical Reasoning
- Difficulty: Easy
- Time target: 40 seconds
Question
Which one does not belong?
3, 5, 7, 9, 11
Options
- 3
- 5
- 9
- 11
Show the answer and the worked solution
Correct answer: C. 9
Worked solution
- All the numbers are odd, so oddness cannot be the rule.
- 3, 5, 7 and 11 are prime; 9 = 3 × 3 is not.
Answer: 9.
The idea
When an obvious property is shared by everything, look for a second one. Being odd is a decoy here.
Shortcut. The digit-sum test shows 9 is divisible by 3 without being 3 itself, so it is the only composite; the rest are prime.
Common trap. 11 — the only two-digit number, a surface feature; primality leaves 9 out alone.
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