Interpreting Function Notation, Graphs, and Context

Alex has modeled his success with crossword puzzles as the following: $$n=4s+6h$$ Where: $$\begin{align}n&=\text{Number of words filled in}\\ s&=\text{Hours of sleep the night before}\\ h&=\text{Number of people helping complete the crossword}\end{align}$$

Which variables are independent?