Solve each application.

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 ƒ(x) = 86.3 ln x - 680, where x is the altitude in feet and ƒ(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 ft 

(b) 4000 ft 

(c) 7000 ft