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: 1, 2, 3, or 6? Number of days: 3, 4, 5, or 6?

After the first three days, the sample size will be half of the initial; therefore, 400g/2 = 200g. However, it is asking how long will it take to decay to 100g, so we will take 200g/2 = 100g, which will take another three days. It will take 2 half-lives, which will encompass 6 days.