The half-life of cobalt-60 is 5.25 yrs. After how long does its activity reduce to about one eight of its original value?
15.75 years
The activity is proportional to the number of undecayed atoms: In each half-life, the remaining sample decays to half of its initial value. Since (12)×(12)×(12)=18, therefore, three half-lives or 15.75 years are required for the sample to decay to 18th its original strength.