SAT Question f718c9cf: Systems of Linear Equations answer and explanation with Desmos solution

Multiplying the first equation in the given system by $2$ yields $10x+28y=90$. Subtracting the second equation in the given system, $10x+7y=27$, from $10x+28y=90$ yields $\left(10x+28y\right)-\left(10x+7y\right)=90-27$, which is equivalent to $10x+28y-10x-7y=63$, or $21y=63$. Dividing both sides of this equation by $21$ yields $y=3$. The value of $x$ can be found by substituting $3$ for $y$ in either of the two given equations. Substituting $3$ for $y$ in the equation $10x+7y=27$ yields $10x+7\left(3\right)=27$, or $10x+21=27$. Subtracting $21$ from both sides of this equation yields $10x=6$. Dividing both sides of this equation by $10$ yields $x=\frac{6}{10}$, or $x=\frac{3}{5}$. Therefore, the value of $xy$ is $\left(\frac{3}{5}\right)\left(3\right)$, or $\frac{9}{5}$. Note that 9/5, 1.8, and 1.80 are examples of ways to enter a correct answer.