Answer
There is $ 1\%$ of surface sunlight at $118$ feet.
The point $(118,1)$ is on the graph.
Work Step by Step
We want to find the x for which $ f(x)=1.$
$ 1=20(0.975)^{x}\quad $ ... $/\div 20$
$\displaystyle \frac{1}{20}=0.975^{x}\quad $ ... $/\ln(...)$
$\displaystyle \ln\frac{1}{20}=x\ln 0.975\quad $ ... $/\div\ln 0.975$
$ x=\displaystyle \frac{\ln(1/20)}{\ln 0.975}\approx 118.325$
There is $ 1\%$ of surface sunlight at $118$ feet.
The point $(118,1)$ is on the graph.