SAT Question ee05c84e: Nonlinear Functions answer and explanation with Desmos solution

Choice C is correct. Adding the like terms $x$ and $0.25x$ yields the equation $f(x)=1.25x(50-x)$. Substituting $20$ for $x$ yields $f(20)=(1.25\times 20)(50-20)$. The product $1.25\times 20$ is equal to $25$, and the difference $50-20$ is equal to $30$. Substituting these values in the given equation gives $f(20)=25\times 30$, and multiplying $25$ by $30$ results in $f(20)=750$.