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A radioactive sample with a half-life of 1 month has the label: 'Activity = 2 micro curies on 1-8-1991. What would be its activity two months earlier?

1. 1.0 micro curie
2. 0.5 micro curie
3. 4 micro curie
4. 8 micro curie

Correct answer: 8 micro curie

Solution

The activity of a radioactive sample doubles for every half-life when going backward in time. Since the half-life is 1 month, going 2 months earlier means doubling the activity twice: 2 μCi → 4 μCi → 8 μCi.

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