An accident at a nuclear power plant has left the surrounding area polluted with radioactive material that decays naturally. The initial amount of radioactive material present is 15 su (safe units), and 5 months later it is still 10 su.
(a) Write a formula giving the amount A(t) of radioactive material (in su) remaining after t months.
(b) What amount of radioactive material will remain after 8 months?
(c) How long—total number of months or fraction thereof—will it be until A = 1 su, so it is safe for people to return to the area?