# Math Help - Half-Life calculation

1. ## Half-Life calculation

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!

2. Let A= the amount of radioactive material remaining at anytime t, you have the following information

Use the following $A = A_0e^{kt}$

at $t=0, A = A_0$ and $t=5780, A = \frac{A_0}{2}$

After this solve for t when $A = 0.05\times A_0$