Odd One Out — In the triples 3–4–14, 56–57–67, 102–103–113, 22–23–13, three end with a prime; one ends with a composite. Identify the exception.

Difficulty: Medium

Correct Answer: 3-4-14

Explanation:


Introduction / Context:
Prime identification within short triples is a common classification trick. Here, the last term is decisive: three are primes; one is composite.



Given Data / Assumptions:

  • Candidates for the third term: 14, 67, 113, 13.
  • Primality checks: 67 (prime), 113 (prime), 13 (prime), 14 (composite = 2 * 7).


Concept / Approach:
Check whether the final number of each triple is prime. If only one is composite, it is the odd one out.



Step-by-Step Solution:
56–57–67 → 67 is prime.102–103–113 → 113 is prime.22–23–13 → 13 is prime.3–4–14 → 14 is composite (2 * 7).



Verification / Alternative check:
Quick divisibility rules: 14 is even → divisible by 2; the others are not divisible by small primes (3, 5, 7) within their bounds, confirming primality.



Why Other Options Are Wrong:

  • 56–57–67: ends with a prime.
  • 102–103–113: ends with a prime.
  • 22–23–13: ends with a prime.


Common Pitfalls:
Overanalyzing the first two terms. The discriminant here is solely the primality of the third term.



Final Answer:
3-4-14

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