The half-life of iodine is 8 days. if 700.00 g sample decays to 43.75, how much time has passed

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. 

C) electrons encircling a nucleus with neutrons and protons

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water