The rate of change is the ratio between to changes.
The first change is the degrees outside and this happens in 5 hours, which is the second change.
However, we need to notice that the 10 degrees is a drop, that is, it decreases. This means that the change in temperature is negative.
So, we have:
[tex]\begin{gathered} r\Rightarrow\, \text{rate of change} \\ \Delta y\Rightarrow\, \text{change in the temperature} \\ \Delta x\Rightarrow\, \text{change in time} \\ r=\frac{\Delta y}{\Delta x} \\ r=\frac{-10\degree}{5h} \\ r=-\frac{10}{5}\degree\/h \\ r=-2\degree\/h \end{gathered}[/tex]
So, the rate of change is -2 °\h, that is, -2 degrees per hour.
