The profit $P(x)$ from selling $x$ units of a product is modeled by the function $P(x)=-0.005x^2+20x$, where $x$ is the number of units sold. The graph of $y=P(x)$ is a parabola that opens downward and intersects the x-axis at two points. The maximum profit occurs at the vertex of this parabola. If one x-intercept is at $x=0$, and the other is at $x=k$, what is the number of units sold that yields the maximum profit?