The question is incomplete, the complete question is;
The half-life of strontium-90 is about 29 years. How many half-lives have elapsed when 6.25% of a strontium- 90 sample remains in the container?
Answer:
4 half lives
Step-by-step explanation:
From;
N/No = (1/2)^t/t1/2
N= amount of strontium-90 present at time t
No= amount of strontium-90 originally present
t= time taken
t1/2 = half life of strontium-90
But N= 0.0625No
Hence;
0.0625No/No = (1/2)^t/29
0.0625 = (1/2)^t/29
(1/2)^4 = (1/2)^t/29
4 = t/29
t = 4 × 29
t= 116 years
Hence four half lives have elapsed