Fermium-253 has a half-life of three days. A sample contains 400 g. How long will it take for the sample to decay to 100 g? Number of half-lives:

Each half-life cuts the sample in half: 400 g becomes 200 g after one half-life, then 100 g after two. Therefore, it takes 2 half-lives to reach 100 g, which is choice B.

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