## Precalculus (6th Edition) Blitzer

There is $3\%$ of surface sunlight at $75$ feet. The point $(75,3)$ is on the graph.
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.