A company's monthly revenue, in dollars, from selling x units of a product is modeled by R(x) = -5x² + 200x. How many units should the…

The revenue function $R(x) = -5x^2 + 200x$ is a downward-opening parabola, so its maximum is at the vertex. The vertex has $x = -\frac{b}{2a} = -\frac{200}{2(-5)} = 20$ units, choice A. The maximum revenue is $R(20) = -2000 + 4000 = 2000$ dollars. Choice B (40) is where revenue returns to zero, the other $x$-intercept, so it is twice the vertex. Choice C (200) is the coefficient of $x$. Choice D (10) comes from using $-\frac{b}{4a}$ or dividing incorrectly. The question asks for the number of units, not the revenue.