How long has the tree been dead?

[Math10]

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.

 

[Joffan]

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]

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.

 

[Math10]

The answer is (tau)*ln(p0/p1)/ln(2).

 

[epenguin]

Right