During an epidemic, the number of people who have never had the disease and who are not immune (they are susceptible) decreases exponentially according to the function ƒ(t) = 15,000e^-0.05t, where t is time in days. Find the number of susceptible people at each time.

(a) At the beginning of the epidemic

(b) After 10 days 

(c) After 3 weeks