Question

The half-life of cobalt-60 is 5 years. How old is a sample of cobalt-60 if only one-eighth of the original sample is still cobalt-60?

5 years
10 years
15 years
20 years

Answer 1

15 years

Reason: 1/8 = (1/2)^3, so three half-lives have passed. With a 5-year half-life, 3 × 5 = 15 years.