Find the odd number out: 6, 9, 15, 21, 24, 28, 30.

Introduction / Context:
Many odd-one-out puzzles can be solved via quick divisibility tests. Here, note that most numbers are divisible by 3 while one is not, making it stand out as the odd term.

Given Data / Assumptions:

• Sequence: 6, 9, 15, 21, 24, 28, 30
• Divisibility by 3 test: digit sum divisible by 3 ⇒ number divisible by 3

Concept / Approach:
Apply the digit-sum test to each term. All but one will pass; the failing number is the outlier.

Step-by-Step Solution:
6 → 6 divisible by 3 ✔9 → 9 divisible by 3 ✔15 → 1+5=6 divisible by 3 ✔21 → 2+1=3 divisible by 3 ✔24 → 2+4=6 divisible by 3 ✔28 → 2+8=10 not divisible by 3 ✖30 → 3+0=3 divisible by 3 ✔

Verification / Alternative check:
Direct division: 28/3 leaves remainder 1; each of the others divides evenly by 3.

Why Other Options Are Wrong:
24/30/21/15 are all multiples of 3 and thus fit the majority pattern.

Common Pitfalls:
Assuming “evenness” is the rule; here 24 and 30 are even yet multiples of 3, so parity alone is not the key classifier.

Final Answer:
28