If you start with 100 grams of a radionuclide, how much will you have after two half-lives have passed?

25 grams.

A half life is the amount of time it takes for an isotope to decay half of itself.

So after one half life, there will be half of the original amount.

100/2=50

After another half life, there will be half of that amount.

50/2 = 25

After first half-life you will have 100/2=50g. After second half-life you will have 50/2=25g.