Radioactive carbon decays exponentially with a half-life of 5,780 years. Determine how long ago a sample of carbon began to decay if there is now 5% of it left.
I'm not sure how to get started to all help is appreciated!
Let A= the amount of radioactive material remaining at anytime t, you have the following information
Use the following $\displaystyle A = A_0e^{kt}$
at $\displaystyle t=0, A = A_0$ and $\displaystyle t=5780, A = \frac{A_0}{2}$
this will help you solve for k.
After this solve for t when $\displaystyle A = 0.05\times A_0$