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A sample of radioactive material A, that has an activity of 10 mCi(1 Ci = 3.7 × 10 10 decays/s), has twice the number of nuclei as another sample of different radioactive material B which has an activity of 20 mCi. The correct choices for half-lives of A and B would then be respectively :

1. 10 days and 40 days
2. 20 days and 10 days
3. 5 days and 10 dyas
4. 20 days and 5 days

Correct answer: 20 days and 5 days

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