Question

A radioactive substance decays exponentially with a half life of 500 years. After 1,500 years, what percent of the radioactive material remains?

Answer 1

There are fancy formulas for this but this calculation is easy.
1,500 years exactly equals 3 half-lives.  After 1 half-life you have 50%, after 2 half-lives, you have 25% and after 3 half-lives 12.5 per cent remains.