In 1990 a fatal disease evolves to which 40 percent of a population of 5 million trees is susceptible. The proportion of susceptible trees which survive for a period of t years beyond 1990 is e^(-t).

(b) How quickly is the disease killing off trees at the start of 1992? (trees per year)

(c) When will the population be reduced to 80 percent of the level in 1990?