Question

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?

Answer 1

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.