SAT Mathematics Practice Test 4

Since the average of 2 numbers is the sum of the 2 numbers divided by 2, the equations $\mathit{x}=\frac{\mathit{m}+9}{9},\mathit{y}=\frac{2\mathit{m}+15}{2}$ and $\mathit{z}=\frac{3\mathit{m}+18}{2}$ are true. The average of x, y, and z is given by $\frac{\mathit{x}+\mathit{y}+\mathit{z}}{3}$ Substituting the preceding expressions in m for each variable gives $\frac{\frac{\mathit{m}+9}{2}+\frac{2\mathit{m}+15}{2}+\frac{3\mathit{m}+18}{2}}{3}$ This fraction can be simplified to $\frac{6\mathit{m}+42}{6}$ or $\mathit{m}+7$ Choices A, C, and D are incorrect and may be the result of conceptual errors or calculation errors. For example, choice D is the sum of x, y, and z, not the average.