The radioactive substance strontium-90 has a half-life of 28 years. in other words, it takes 28 years for half of a given quantity of strontium-90 to decay to a non-radioactive substance. the amount of radioactive strontium-90 still present after t years is modeled by the expression 80*2^t/28 grams. evaluate the expression for t = 0, t = 28 and t = 56. what does each value of the expression represent?