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?

This is a relatively easy half life calculation.
After one half-life period, half of the isotope remains.
After two half-life periods, one quarter of the isotope remains.
That being the case, we see this will take 2 half-life periods or 6 days.