SAT Question 5518885d: Linear Equations In Two Variables answer and explanation with Desmos solution

Choice B is correct. It's given that the equation $x+y=1,440$ represents the number of minutes of daylight, $x$, and the number of minutes of non-daylight, $y$, on a particular day in Oak Park, Illinois. It's also given that this day has $670$ minutes of daylight. Substituting $670$ for $x$ in the equation $x+y=1,440$ yields $670+y=1,440$. Subtracting $670$ from both sides of this equation yields $y=770$. Therefore, this day has $770$ minutes of non-daylight.