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April 25, 2015

Homework Help: Math/Radioactive Dating

Posted by Lena on Saturday, February 19, 2011 at 5:56pm.

My teacher said that if the ratio of mass (in a rock) for 40K and 40Ar was 1:3 then the age of the rock would be 2.5 billion years. I don't understand how he concluded that to be the age. The only thing he knew was that the half-life of K40 = 1.25 billion year. This is my thinking.


1/3 = 1/2 ^ (t/(1.25x10^9))
log(1/3)/log(1/2) = t/(1.25*10^9)
1.58=t/(1.25*10^9)
t= 1.58 *(1.25*10^9)
t= 1.98*10^9

I don't understand where he came up with 2.5 billion years as the age of the rock.

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