How old is a piece of wood that has lost 90% of its carbon-14?

I know that there is only a presence of 10% but don't know how to find the decay rate (k).

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- Nov 17th 2009, 09:47 AMlittlesohiExponential Decay
How old is a piece of wood that has lost 90% of its carbon-14?

I know that there is only a presence of 10% but don't know how to find the decay rate (k). - Nov 17th 2009, 10:45 AMearboth
1. The half-life time of carbon-14 is 5730 years.

2. Use the equation:

$\displaystyle A(t)=A_0 \cdot e^{k \cdot t}$ to calculate k,

where A(t) is the present amount and $\displaystyle A_0$ the initial amount.

3. $\displaystyle \frac12 A_0 = A_0 \cdot e^{k \cdot 5730}~\implies~k= \dfrac{\ln\left(\frac12\right)}{5730}$

4. Solve the following equation for t:

$\displaystyle \frac1{10} A_0=A_0 \cdot e^{\frac{\ln\left(\frac12\right)}{5730} \cdot t}$

__Spoiler__: