According to the definition the Half-life is the time it takes for something to become half of the initial value.
If the isotope has a half-life of 10 years, means that after 10 years the amount of isotope you will have is half of the 100 g
After 10 years you will have 50g.
Now if another 10 years pass, then the initial value at 10 years will be 50 g and after another 10 years you will have 25 g.
After 20 years you will have 25 g.