SAT Mathematics Practice Test 5

Use substitution to create a one-variable equation that can be solved for x. The second equation gives that $\mathit{y}=2\mathit{x}.$ Substituting $2\mathit{x}$ for $\mathit{y}$ in the first equation gives $\frac{1}{2}\left(2\mathit{x}+2\mathit{x}\right)=\frac{21}{2}$ . Dividing both sides of this equation by $\frac{1}{2}$ yields to $\left(2\mathit{x}+2\mathit{x}\right)=21$. Combining like terms results in $4\mathit{x}=21$. Finally, dividing both sides by 4 gives $\mathit{x}=\frac{21}{4}=5.25$.Either $\frac{21}{4}$ or $5.25$ can be gridded as the correct answer.