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.
The Attempt at a Solution
I set the equation up so that 8=A0e100k and figured I could solve from there taking the ln but that gives me ln8/A0=100k... I still have two variables so I can't solve. How do I fix this? Can I plug in 20 for k?