1. The problem states that $y$ is inversely proportional to $x$, and we know $y=3$ when $x=15$. We need to find a formula for $y$ in terms of $x$. 2. When a variable $y$ is inversely proportional to $x$, it means: $$y \propto \frac{1}{x}$$ which can be written as: $$y = \frac{k}{x}$$ where $k$ is the constant of proportionality. 3. To find $k$, substitute the known values $y=3$ and $x=15$: $$3 = \frac{k}{15}$$ Multiply both sides by 15: $$3 \times 15 = k$$ $$k = 45$$ 4. Therefore, the formula for $y$ in terms of $x$ is: $$y = \frac{45}{x}$$ This formula means that for any value of $x$, $y$ can be found by dividing 45 by $x$. Final answer: $$\boxed{y = \frac{45}{x}}$$