Astatine is a radioactive chemical element that was first produced at the University of California, Berkeley in 1940. The half-life of astatine is 8 hours. Write an exponential function that models the mass in grams y remaining from a 560-gram sample after t hours. When will approximately 198 grams remain in the sample? How many grams will remain after 3 days? Express your answer as a decimal rounded to the nearest hundredth.