How much of a 400g sample remains after 4 years?

After four years, 100 grams of this isotope will be left. Since someone has asked for how it is worked out, I will do my best to show that. So, the original sample is 400 grams. The question states that this particular isotope has a half life of two years. A half life is the amount of time it takes half of a radioactive isotope to decay and become safe. So, this is concerning what is left after two half lives.

After two years, the first half life, the sample will have 200 grams left. Another two years pass, and another 100 grams decays away (half the second sample). That leaves 100 grams of the original sample after four years.