(A) 10 Million Years(X)
(B) 20 Million years (
)
(C) 30 Million years(X)
(D) 40 Million years(X)
Answers: 20 Million years
An igneous rock contains a radioactive isotope that has a half life of 10 million years. Careful analysis shows that only one quarter of the original concentration of the parent isotope is left. How old is this igneous rock?
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