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?
Answer
A
15 years
B
5 years
C
20 years
D
10 years
Explanation
One-eighth of the original amount is 1/2 × 1/2 × 1/2, so three half-lives have passed. Each half-life is 5 years, giving 3 × 5 = 15 years. The sample is 15 years old, choice A.
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