-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

Question

-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

Quizplus · Accepted Answer

The rock originally had 20 grams of ⁴⁰K and now has 5 grams. This means it has gone through two half-lives (20g to 10g to 5g). Since each half-life is 1.3 billion years, the rock is 2.6 billion years old (1.3 billion years x 2).

-The Diagram Illustrates the Radioactive Decay of Potassium (⁴⁰K),which Has