1. The problem statement, all variables and given/known data A tree contains a known percentage p0 of a radioactive substance with half-life tau. When the tree dies the substance decays and isn't replaced. If the percentage of the substance in the fossilized remains of such a tree is found to be p1, how long has the tree been dead? 2. Relevant equations I don't know what formula to use but I know that I have to solve for t in the formula. So I just need to know the formula. 3. The attempt at a solution p1=(1/2)^(-t/tau) This is all I can come up at this moment. I don't know if this is the right formula.