A certain radioactive nuclide has a half-life of $200.0 \mathrm{~s}$. A sample containing just this one radioactive nuclide has an initial activity of $80,000.0 \mathrm{~s}^{-1}$. (a) What is the activity $600.0$ s later? (b) How many nuclei were there initially? (c) What is the probability per second that any one of the nuclei decays?