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.

It feels wrong to not do a question about my own profile picture, so here it goes.
This is kinda inspired by Kumon Level O 199b. Somewhat. If you have done this question you know that it was super weird.

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