If the half life of an isotope is 20 years, how much of the original amount will remain after 20 years has passed?

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.

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it