Classification – Odd one out (multiples of 13) In the following set, three numbers are multiples of 13 while one is not. Identify the non-multiple of 13 and mark it as the odd one out.

Introduction / Context:
Recognizing multiples of medium primes (like 13) is a useful classification skill. Many exam sets include two or three crisp multiples and a single non-multiple to create a unique outlier.

Given Data / Assumptions:

• Candidates: 91, 143, 247, 215
• Property checked: divisibility by 13

Concept / Approach:
Recall familiar multiples: 13 * 7 = 91, 13 * 11 = 143, 13 * 19 = 247. If three values match this pattern, the leftover non-multiple is the odd element.

Step-by-Step Solution:
91 = 13 * 7 → multiple of 13.143 = 13 * 11 → multiple of 13.247 = 13 * 19 → multiple of 13.215 = 5 * 43 → not a multiple of 13.

Verification / Alternative check:
Divide directly by 13: 91/13 = 7, 143/13 = 11, 247/13 = 19 (integers). 215/13 = 16 remainder 7, so not an integer. Therefore, 215 is the unique non-multiple.

Why Other Options Are Wrong:

• 91: Multiple of 13.
• 143: Multiple of 13.
• 247: Multiple of 13.
• None of these: There is a single non-multiple (215).

Common Pitfalls:
Confusing “ends with 5” as an indicator of special status. Ending in 5 reveals divisibility by 5, not by 13. Always test the target factor explicitly.

Final Answer:
215