SAT Mathematics Practice Test 3

Dividing each side of the equation $3\mathit{r}=18$ by 3 gives $\mathit{r}=6$.
Substituting 6 for r in the expression $6\mathit{r}+3$ gives $6\left(6\right)+3=39$.
Alternatively, the expression $6\mathit{r}+3$ can be rewritten as $2\left(3\mathit{r}\right)+3$.
Substituting $18$ for $3\mathit{r}$ in the expression $2\left(3\mathit{r}\right)+3$ yields $2\left(18\right)+3=36+3=39.$
Choice A is incorrect because $6$ is the value of r; however, the question asks for the value of the expression $6\mathit{r}+3$.
Choices B and C are incorrect because if $6\mathit{r}+3$ were equal to either of these values, then it would not be possible for $3\mathit{r}$ to be equal to $18$, as stated in the question.