(d) Find the rate of growth after 8 hours. (Round your answer to three decimal places.) billion cells per hour (e) When will the population.reach 20,000 cells? (Round your answer to two decimal places.) hr Submit Answer [-/25 Points] DETAILS SCALCET8 3.8.010. A sample of a radioactive substance decayed to 96.5% of its original amount after a year. (Round your answers to two decimal places.) (a) What is the half-life of the substance? yr (b) How long would it take the sample to decay to 75% of its original amount? yr Submit Answer