Two-variable data: Models and scatterplots SAT question

The correct answer is $\frac{9}{2}$. It's given that the scatterplot shows the relationship between the length of time $y$, in hours, a certain bird spent in flight and the number of days after January $11$, $x$. Since January $13$ is $2$ days after January $11$, it follows that January $13$ corresponds to an x-value of $2$ in the scatterplot. In the scatterplot, when $x=2$, the corresponding value of $y$ is $6$. In other words, on January $13$, the bird spent $6$ hours in flight. Since January $15$ is $4$ days after January $11$, it follows that January $15$ corresponds to an x-value of $4$ in the scatterplot. In the scatterplot, when $x=4$, the corresponding value of $y$ is $15$. In other words, on January $15$, the bird spent $15$ hours in flight. Therefore, the average rate of change, in hours per day, of the length of time the bird spent in flight on January $13$ to the length of time the bird spent in flight on January $15$ is the difference in the length of time, in hours, the bird spent in flight divided by the difference in the number of days after January $11$, or $\frac{15-6}{4-2}$, which is equivalent to $\frac{9}{2}$. Note that 9/2 and 4.5 are examples of ways to enter a correct answer.