Odd One Out — Among 5:9, 7:11, 13:17, 29:31, select the pair that is not a pair of primes.

Introduction / Context:
Prime versus composite recognition is central to many number-classification items. Three options are prime-prime pairs; one includes a composite number.

Given Data / Assumptions:

• Primes among the listed numbers: 5, 7, 11, 13, 17, 29, 31.
• 9 is composite (3 * 3).

Concept / Approach:
Check each member of the pair for primality; the pair containing a composite is the outlier.

Step-by-Step Solution:
5:9 — 5 is prime, 9 is composite → mismatched pair.7:11 — both prime.13:17 — both prime.29:31 — both prime.

Verification / Alternative check:
All-prime pairs persist even if you test differences (4, 4, 2). The single composite presence is the decisive discriminator.

Why Other Options Are Wrong:

• 7:11 — prime pair.
• 13:17 — prime pair.
• 29:31 — prime pair.

Common Pitfalls:
Confusing “odd” with “prime.” Not all odd numbers are prime; 9 is a counterexample.

Final Answer:
5 : 9