Classification – Odd one out (unique prime among composites) Three of these numbers are composite. Identify the only prime number and mark it as the odd one out.

Introduction / Context:
Quick prime tests differentiate primes from obvious composites. This is essential in classification problems aimed at speed and accuracy.

Given Data / Assumptions:

• Options: 143, 133, 131, 87
• We check small divisors to separate prime from composites.

Concept / Approach:
Factor or test divisibility: 143 = 11 * 13; 133 = 7 * 19; 87 = 3 * 29. These are composite. 131 must be tested for divisibility by primes ≤ sqrt(131) ≈ 11.44.

Step-by-Step Solution:
143 → 11 * 13 → composite.133 → 7 * 19 → composite.87 → 3 * 29 → composite.131 → not divisible by 2, 3, 5, 7, 11 → prime.

Verification / Alternative check:
Perform trial division: None of the primes 2, 3, 5, 7, 11 divides 131; thus it is prime.

Why Other Options Are Wrong:

• 143: Composite.
• 133: Composite.
• 87: Composite.
• None of these: Exactly one prime exists (131).

Common Pitfalls:
Assuming two-digit numbers with 1 or 3 are likely prime. Always check small prime divisibility.

Final Answer:
131