How do you slove this half life problem?

Question

Potassium-40 will decay into the element argon-40. It's half life is about 1.3 billion years. If you found a sample that was 12.5% Potassium-40 and 87.5% argon-40, how old would you estimate that this sample was?

Answer 1

12.5 is 1/8th, or 1/(2^3), which means that three half-lives have elapsed.

Therefore I would estimate that the sample is 3.9 billion years old