If the half life of an isotope is 20 years, then half of the original amount will remain after 20 years has passed.
That's what "half-life" means.
Similarly, after ANOTHER 20 years, 1\4 of the original amount remains.
And after ANOTHER 20 years, 1\8 of the original amount remains.
And after ANOTHER 20 years, 1\16 of the original amount remains.
And after ANOTHER 20 years, 1\32 of the original amount remains.
And after ANOTHER 20 years, 1\64 of the original amount remains.
And after ANOTHER 20 years, 1\128 of the original amount remains.
