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## Homework Statement

A radioactive substance has a half-life of 20 years. If 8 mg of the substance remains after

100 years, find how much of the substance was initially present.

## Homework Equations

A=A

_{0}e

^{kt}

## The Attempt at a Solution

I set the equation up so that 8=A

_{0}e

^{100k}and figured I could solve from there taking the ln but that gives me ln8/A

_{0}=100k... I still have two variables so I can't solve. How do I fix this? Can I plug in 20 for k?