Question

iodine 131
half life 8 days
20 mg iodine 131 is presnt in store, it was 48 days ago from laboratory
finf quantity of iodine 131 left after this period

Answer 1

one approach
1/2 life is time it takes for 1/2 the mass to "disappear"
formula is (don't remember)
48 days ago means 48/ 8 = 6 times
20/2 = 10 after 8 days
10/2 = 5 16 days
5/2 = 2.5 24 days
2.5/2 = 1.25 32 days
1.25/2 = 0.625 40 days
0.625/2 = 0.3125g after 48 days

Answer 2

thnkoo :)