SAT Mathematics Practice Test 3

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