Answer
5.25 years.
Work Step by Step
The sample mass has been decreased by a factor of four.
This means that the sample has been decreased to half its size twice. Make sure that a half-life is the amount of time it takes for half of a radioactive sample to decay.
So, we know that the sample had gone through two half-lives in 10.5 years (from .800 g to 0.400 g, and then from 0.400 g to 0.200 g). Hence, the half-life of cobalt-60 is 5.25 years.
