# Geology

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I need help with a radiometric dating equation. I am trying to determine the age of a sample that has a 40AR/40K ratio of 4.5. I understand the equation of D=Do+N(e^.693t-1). I understand I would set Do to zero. But what is D? I went to geology tutoring and she said to plug 4.5 in there but I don't think that is right since it is the ratio but I don't know. I don't understand. Please help. Thank you.

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