Strontium 90 is a radioactive material that decays according to the function A t Age 0 024 amount present and A is the amount present at time t in years Assume that a scientist has of strontium 90 a What is the decay rate of strontium 90 b How much strontium 90 is left after 20 years c When will only 200 grams of strontium 90 be left d What is the half life of strontium 90 a The decay rate of strontium 90 is Type an integer or a decimal Include the negative sign for the decay rate