SAT Question 9db5b5c1: Systems of Linear Equations answer and explanation with Desmos solution

Choice C is correct. It's given that $4x=20$ and $-3x+y=-7$ is a system of equations with a solution $\left(x,y\right)$. Adding the second equation in the given system to the first equation yields $4x+(-3x+y)=20+\left(-7\right)$, which is equivalent to $x+y=13$. Thus, the value of $x+y$ is $13$.