In the central Sierra Nevada (a mountain range in California), the percent of moisture that falls as snow rather than rain is approximated reasonably well by
$$
f(x)=86.3 \ln x-680
$$
where $x$ is the altitude in feet and $f(x)$ is the percent of moisture that falls as snow. Find the percent of moisture, to the nearest tenth, that falls as snow at each altitude.
(a) $3000 \mathrm{ft}$
(b) $4000 \mathrm{ft}$
(c) $7000 \mathrm{ft}$