SAT Question b52e5b6f: Linear Equations In Two Variables answer and explanation with Desmos solution

Choice C is correct. Let $d$ represent the mass, in grams, of vitamin D in the mixture, and let $c$ represent the mass, in grams, of calcium in the mixture. It's given that the mixture consists of only vitamin D and calcium and that the total mass of the mixture is $150$ grams. Therefore, the equation $d+c=150$ represents this situation. It's also given that the mass of vitamin D in the mixture is $50$ grams. Substituting $50$ for $d$ in the equation $d+c=150$ yields $50+c=150$. Subtracting $50$ from both sides of this equation yields $c=100$. Therefore, the mass of calcium in the mixture is $100$ grams.