# Radioactive

**Algebra**Level 3

Radioactive substances decay over time and lose mass at a rate that is proportional to the mass remaining. If after 13000 years, 1/4 of the mass is left, how much mass is left after another 6500 years?

**Bonus 1**: Which isotope am i dealing with here?

**Bonus 2**: Solve this using Calculus, with B as the constant of proportion. That's how it was solved in the Solution Book. That's why I don't trust it.