SAT Question adb0c96c: Systems of Linear Equations answer and explanation with Desmos solution

Subtracting the second equation in the given system from the first equation yields $\left(24x+y\right)-\left(6x+y\right)=48-72$, which is equivalent to $24x-6x+y-y=-24$, or $18x=-24$. Dividing each side of this equation by $3$ yields $6x=-8$. Substituting $-8$ for $6x$ in the second equation yields $-8+y=72$. Adding $8$ to both sides of this equation yields $y=80$.

Alternate approach: Multiplying each side of the second equation in the given system by $4$ yields $24x+4y=288$. Subtracting the first equation in the given system from this equation yields $\left(24x+4y\right)-\left(24x+y\right)=288-48$, which is equivalent to $24x-24x+4y-y=240$, or $3y=240$. Dividing each side of this equation by $3$ yields $y=80$.