
Answer:
3.5 milligram
Explanation:
From;
N/No = (1/2)^t/t1/2
Where;
N = amount of Po-210 that remains unchanged after 414 days
No = 28.0 milligram of Po-210 originally present
t = 414 days
t1/2 = half life of Po-210 = 138 days
Substituting values;
N/28 = (1/2)^414/138
N/28 = (1/2)^3
N/28 = 1/8
8N = 28
N = 28/8
N = 3.5 milligram