In a direct variation the constant of proportionality can be found by the equation:
[tex]y=kx[/tex]
where x and y are the variables. In this case, for the first data we have that x=2 hours and y=25 miles, then:
[tex]\begin{gathered} 25=2k \\ k=\frac{25}{2} \\ k=12.5 \end{gathered}[/tex]
Therefore the constant of proportionality is 12.5
