## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$y=\frac{1}{5} x$
$y$ varies directly with $x$, then $y=kx$ where $k$ is the constant of proportionality. Since $y=2$ when $x=10$, then substituting these into the equation above gives: $$2=k (10)\\ \frac{2}{10}=k\\ k=\frac{1}{5}$$ Therefore, the equation is y$=\frac{1}{5}x$ is the required general formula.