SAT Question 13294295: Linear Functions answer and explanation with Desmos solution

Choice B is correct. For the graph shown, the x-axis represents time, in seconds, and the y-axis represents the number of candy bars wrapped. The slope of a line in the xy-plane is the change in $y$ for each $1$-unit increase in $x$. It follows that the slope of the graph shown represents the estimated number of candy bars the machine wraps with a label per second. The slope, $m$, of a line in the xy-plane can be found using any two points, $\left(x_1,y_1\right)$ and $\left(x_2,y_2\right)$, on the line and the slope formula $m=\frac{y_2-y_1}{x_2-x_1}$. The graph shown passes through the points $\left(0,0\right)$ and $\left(2,80\right)$. Substituting these points for $\left(x_1,y_1\right)$ and $\left(x_2,y_2\right)$, respectively, in the slope formula yields $m=\frac{80-0}{2-0}$, which is equivalent to $m=\frac{80}{2}$, or $m=40$. Therefore, the estimated number of candy bars the machine wraps with a label per second is $40$.