Asample of a radioactive element has a mass of 80 g. how much parent and daughter materials are in the sample after two half-lives?

The half-life is the time it takes for the radioactive sample to halve.

The initial sample is 80 g. After one half-life, the initial sample (parent material) will be half of its initial amount, i.e. 40 g. The amount of daughter material will be 80-40=40 g.

After two half-lives, the amount of parent material will halve again: therefore, only 20 g will be left in the parent material, and the amount of daughter material will be 80-20=60 g.

So the answer is: after 2 half-lives, we have 20 g of parent material and 60 g of daughter materia.

conservation of mass

mass before something happens = mass afterwards