Two-variable data: Models and scatterplots SAT question

The correct answer is $\frac{1}{2}$. For the graph shown, $x$ represents time, in seconds, and $y$ represents momentum, in newton-seconds. Therefore, the average rate of change, in newton-seconds per second, in the momentum of the object between two x-values is the difference in the corresponding y-values divided by the difference in the x-values. The graph shows that at $x=2$, the corresponding y-value is $6$. The graph also shows that at $x=6$, the corresponding y-value is $8$. It follows that the average rate of change, in newton-seconds per second, from $x=2$ to $x=6$ is $\frac{8-6}{6-2}$, which is equivalent to $\frac{2}{4}$, or $\frac{1}{2}$. Note that 1/2 and .5 are examples of ways to enter a correct answer.