Answer:
21.2 years
Explanation:
If half life is 5.3 years, that means activity will be 50% every 5.3 years.
12,400 x 50% = 6,200
6,200 x 50% = 3,100
3,100 x 50% = 1,550
1,550 x 50% = 775
4 half lives, so 4 half lives x 5.3 years/half life = 21.2 years
