Chapter 1 - Linear Functions and Change - 1.1 Functions and Function Notation - Exercises and Problems for Section 1.1 - Exercises and Problems - Page 6: 8

Independent variable: $n$. dependent variable: $A$.

The number of gallons of paint $n$ represents an arbitrary input. Thereby, it is our independent variable. The area $f(A)$ of the walls is represented an arbitrary output. Since $f$ is a function describing the area in dependence on $A$, our dependent variable is $A$.