Posted by **Lena** on Saturday, February 19, 2011 at 2:12pm.

How would I calculate the age of a rock only given the amount of each in grams and half-life?

40K decays to 40Ar.

Half life of 40K is 1.25 billion years.

40K = 20 grams

40Ar= 5 grams

I think the formula is

Final Amount of 40K = Initial Amount of 40K * (1/2)^(time/half-life)

If I plug in the values I still don't have a value for time. And where would the grams of 40Ar fit in?

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