Potassium 40 (K-40) is a radioisotope that decays to form argon (Ar-40) with a half-life of 1.3 billion years.

Potassium 40 (K-40) is a radioisotope that decays to form argon (Ar-40) with a half-life of 1.3 billion years. Initially, igneous rock contains K-40 but no Ar-40; Ar-40 accumulates inside the rock as K-40 decays. A sample of igneous rock that is 1.3 billion years old will contain atoms of K-40 and Ar-40 in what ratio?