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January 30, 2012 5.8 Exponential Growth and Decay January 30, 2012 1. Iodine 131 is a radioactive material that decays according to the function where A 0 is the initial amount present at time t (in days). Assume that a scientist has a sample of 100 grams of iodine 131. (a) What is the decay rate of iodine 131? (b) How much iodine 131 is left after 9 days? (c) When will 70 grams of iodine 131 be left? (d) What is the halfâ€“life of iodine 131? January 30, 2012 2. A culture of bacteria obeys the law of uninhibited growth. (a) If N is the number of bacteria in the culture and t is the time in hours, express (b) If 500 bacteria are present initially and there are 800 after one hour, how many will be present in the culture after 5 hours? (c) How long is it until there are 20,000 bacteria?