8. A rock contains a radioactive substance which is decaying. The mass of the rock, \(m\) grams, at time \(t\) years after initial observation is given by $$m = 400 + 80 \mathrm { e } ^ { - k t }$$ where \(k\) is a positive constant.
Given that the mass of the rock decreases by \(0.2 \%\) in the first 10 years, find

1. the value of \(k\),
2. the value of \(t\) when \(m = 475\),
3. the rate at which the mass of the rock is decreasing when \(t = 100\).