# How long has the tree been dead?

## Homework Statement

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?

## Homework 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.

## 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.

You can use a positive exponent with (1/2) as your base or you can use a negative exponent with 2 as the base. And p0 should be in there somewhere.

Math10
I got it! Thanks for the help!

epenguin
Homework Helper
Gold Member
I got it! Thanks for the help!

If you have got it why not tell us the answer? The last poster who told me they'd got it, when I asked, hadn't, and I think this is true of quite a lot who say that.