Question

If you have 20 grams of parent material, and one half-life goes by, how much parent material do you have remaining?

Answer 1

Half-life is generally the time it takes for a substance to loose half of its mass - but its not half of the time it takes the substance to loose all its mass- this is typically a lot longer.

So, after a half life, half of the mass will be gone, that is 20/2=10, 10 grams.

another 10 grams will be remaining also.