Classification – Odd one out (multiples of 7) Three of the following integers are multiples of 7. Identify the number that is not divisible by 7 and mark it as the odd one out.

Introduction / Context:
Small-prime multiples (like 7) are frequent in classification items. A quick recall of 7-tables helps isolate the non-multiple instantly.

Given Data / Assumptions:

• Set: 21, 55, 63, 49
• Property target: divisibility by 7

Concept / Approach:
Confirm each value against 7: 21 = 7 * 3, 63 = 7 * 9, 49 = 7 * 7. The remaining value fails the test and becomes the outlier.

Step-by-Step Solution:
21 → 7 * 3 → multiple.63 → 7 * 9 → multiple.49 → 7 * 7 → multiple.55 → 5 * 11 → not a multiple of 7.

Verification / Alternative check:
Direct division 55/7 gives remainder 6, confirming non-divisibility.

Why Other Options Are Wrong:

• 21: Clean multiple of 7.
• 63: Clean multiple of 7.
• 49: Clean multiple of 7.
• None of these: Exactly one non-multiple exists (55).

Common Pitfalls:
Confusing “ends with 1 or 3 or 9” as a rule. Ending digit patterns do not determine 7-divisibility; use factors or division.

Final Answer:
55