Set D to be the distance in miles and t the time in minutes. Notice that the graph goes across (0,0) and (15,3); thus, the equation of the line is given by the formula below
[tex]\begin{gathered} (x_1,y_1),(x_2,y_2) \\ \Rightarrow y-y_1=(\frac{y_2-y_1}{x_2-x_1})(x-x_1) \end{gathered}[/tex]In our case,
[tex]\begin{gathered} (0,0),(15,3) \\ \Rightarrow D-0=(\frac{3-0}{15-0})(t-0) \\ \Rightarrow D=\frac{3}{15}t=\frac{1}{5}t \\ \Rightarrow D=\frac{1}{5}t \end{gathered}[/tex]