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

Multiple Choice

The diagram below 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?
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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