Answer:
32 days
Explanation:
Half life is the amount of time it takes to get to half of the starting concentration / amount.
Starting amount = 250 g
Half life = 8 days
Amount left = 15.625 g
How many days ? That is, how many half lives....
After first Half life;
Amount left = 250 / 2 = 125 g
After second half life;
Amount left = 125 / 2 = 62.5 g
After third half life;
Amount left = 62.5 / 2 = 31.25 g
After fourth half life;
Amount left = 31.25 g / 2 = 15.625 g
That is the radioactive iodine - 131 went though four half lives.
How many days = Number of half lives * Half life
How many days = 4 * 8 = 32 days
