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?

Each half-life leaves half of the cobalt-60. After three half-lives, the remaining fraction is 1/2 × 1/2 × 1/2 = 1/8. Three half-lives of 5 years each take 3 × 5 = 15 years, so the sample is 15 years old (choice C).

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