Its simple, just make a chart. You start off with 700.0g of Iodine in time of 0 days. You want to divide that number by 2 (cause its the half - life). That number will represent 8 days. Divide the x grams of Iodine again, and you will get the half life of 16 days. Keep on doing that until you get 43.75 grams. Use a calculator if you need to.