Pick the odd number: Three numbers are divisible by 9; one is not. Identify the number that is NOT divisible by 9.

Introduction / Context:
Divisibility rules help quickly classify numbers. A number is divisible by 9 if the sum of its digits is divisible by 9. Three of the options satisfy this; one does not.

Given Data / Assumptions:

• Options: 72, 45, 81, 28.
• Rule: If digit sum is a multiple of 9, the number is divisible by 9.

Concept / Approach:
Compute digit sums and check divisibility by 9 for each option.

Step-by-Step Solution:

Verification / Alternative check:
Perform direct division: 72/9 = 8, 45/9 = 5, 81/9 = 9, but 28/9 is not an integer.

Why Other Options Are Wrong:
They are divisible by 9 and hence do not answer the prompt.

Common Pitfalls:
Forgetting or misapplying the digit-sum rule, especially under time pressure.

Final Answer:
28 is not divisible by 9 and is the odd number.