Answer
$0.045$ cm per year
Work Step by Step
The average growth rate of a function $f$ on the interval $(a,b)$ is:
$$\begin{align}\text{Average growth rate}=\dfrac{f(b)-f(a)}{b-a}.\end{align}$$
Identify the elements $a,$, $b$, $f(a)$ and $f(b)$:
$$\begin{align*}
a&=30\\
b&=110\\
f(a)&=11.9\\
f(b)&=15.5.
\end{align*}$$
Calculate the average rate of growth for the given function:
$$\dfrac{15.5-11.9}{110-30}=\dfrac{3.6}{80}=0.045.$$
So the average growth rate is $0.045$ cm per year.
