To represent the statement "The sum of 4.5 and one-third a number is equal to at most 38.9" mathematically, we can write the inequality where \( n \) is the number:
\[ \frac{1}{3}n + 4.5 \leq 38.9 \]
This indicates that the left side (the sum of 4.5 and one-third of \( n \)) must be less than or equal to 38.9.
Therefore, the correct response is:
\[ \frac{1}{3}n + 4.5 \leq 38.9, \quad \text{where } n \text{ is equal to the number.} \]