I'm stuck on a half-life problem.

The formula is A=A_{0}(1/2)^{t/k}where k is the half-life. I am given the half-life and A/A_{0}but I have no idea how I'm supposed to go from that to finding t. The book does not give me an example, the homework website just leaps instantly from that formula to the answer for a sample problem. I have been using natural log as we were using it earlier to do other half-life problems but the natural log doesn't give me any help so obviously I'm not clear on the steps. It's most likely something so basic that I cannot remember it.