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

Problem restatementFrom 6, 9, 15, 21, 24, 28, 30, determine the single term that does not share the common divisibility property.

Given data

• Sequence: 6, 9, 15, 21, 24, 28, 30

Concept / ApproachMost terms look like multiples of 3. Use the sum-of-digits test for divisibility by 3.

Step-by-step validation 6 → 6 (sum 6) → divisible by 3 9 → 9 (sum 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

Exactly one term (28) is not divisible by 3.

Verification / AlternativeDirect division confirms: 28 ÷ 3 = 9 remainder 1 (not a multiple of 3).

Common pitfalls

• Relying on evenness: several terms are even, so parity does not isolate a unique outlier.

Final Answer28