A certain nuclear power plant produces radioactive waste in the form of strontium-90 at the constant rate of 500 pounds per year. The waste decays exponentially with a half-life of 28 years. How much of the radioactive waste from the nuclear plant will be present after 140 years? They give a hint. Think of this as a survival and renewal problem.

I got k=-.025 and if I plug this into the radioactive decay formula I get 1/32. Is this correct? It is nothing like any of the other problems in this section of my book.
sej