The Diagram Illustrates the Radioactive Decay of Potassium (⁴⁰K), Which

The diagram illustrates the radioactive decay of potassium (⁴⁰K) , which has a half-life of 1.3 billion years. If a lunar rock is found that currently contains 5 grams of ⁴⁰K and it is determined that the sample contained 20 grams when it was formed, how old is the lunar rock?

A) 2.6 billion years
B) 3.9 billion years
C) 5.2 billion years
D) 6.5 billion years