Half life is the time period in which half of the initial amount of an element will decay into a new element due to instability. For I-131, the half life is 8 days.
From 200 to 25 grams, let's see how many half lives are there:
200g -> 100g
100g -> 50g
50g -> 25g
Therefore for 200g of I-131 reach 25 grams, 3 half lives are necesseray, 3 half lives or 24 days
