A; rate of change is the same as slope, so, using the slope formula, [tex]m= \frac{y_{2} - y\1_{1} }{ x_{2} - x_{1} } [/tex], where [tex]m[/tex] is the slope and [tex] x_{1} [/tex], [tex] x_{2} [/tex], [tex] y_{1} [/tex], and [tex] y_{2} [/tex] are the [tex]x[/tex] and [tex]y[/tex] values.
Plug in [tex]2[/tex] points for [tex]x[/tex] and [tex]y[/tex], water level being the [tex]y[/tex] and time being the [tex]x[/tex] since we're looking for the answer in ft/hr-- you can choose any two, but from the same row.
[tex]m= \frac{ 30-50}{4-0} [/tex]
[tex]m= \frac{-20}{4} [/tex]
[tex]m= \frac{-5}{1} [/tex]
[tex]m= -5[/tex] ft/hr
