The radiation R (t) in a substance decreases at a rate proportional to the amount present, or dR/dt = kR where k is a constant and t is measured in years. The initial amount of radiation
is 7200 rads. After three years the radiation has declined to 450 rads.
(a) Express R as a function of t.
(b) Find when the radiation will drop below 30 rads.
(c) Find the half-life of this substance