Strontium 90 is a radioactive material that decays according to the function upper a left parenthesis t right parenthesis equals upper a 0 e superscript negative 0.0244 t baseline comma where upper a 0 is the initial amount present and a is the amount present at time t (in years). assume that a scientist has a sample of 400 grams of strontium 90. (a) what is the decay rate of strontium 90? (b) how much strontium 90 is left after 10 years? (c) when will only 300 grams of strontium 90 be left? (d) what is the half-life of strontium 90?