The Diagram Below Illustrates the Radioactive Decay of Potassium

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

A) 6.5 billion years
B) 19.5 billion years
C) 2.6 billion years
D) 3.9 billion years
E) 4.6 billion years