Geology

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How do I do the foll9owing example?
K-40 - A-40 (1.25 billion years)
After careful analysis, a geochronologist determines that an unweathered, unmetamorphosed mineral sample contains 7 trillion atoms of the radioactive element K-40 and 105 trillion atoms of its decay product A-40. The number of years ago that the sample formed is:

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    Anyone I need HELP with this ASAP!!!

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