A certain nuclear power plant produces radioactive waste in the form of strontium-90

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? (hint: Think of this as a survival and renewal problem)