Answer
There is $ 3\%$ of surface sunlight at $75$ feet.
The point $(75,3)$ is on the graph.
Work Step by Step
We want to find the x for which $ f(x)=3.$
$ 3=20(0.975)^{x}\quad $ ... $/\div 20$
$\displaystyle \frac{3}{20}=0.975^{x}\quad $ ... $/\ln(...)$
$\displaystyle \ln\frac{3}{20}=x\ln 0.975\quad $ ... $/\div\ln 0.975$
$ x=\displaystyle \frac{\ln(3/20)}{\ln 0.975}\approx 74.932\approx 75$ feet.
There is $ 3\%$ of surface sunlight at $75$ feet.
The point $(75,3)$ is on the graph.
