If you started with 100 grams of Potassium-40 how much would reamin Potassium-40 after 3.9 x 10 to the 9th years?

Answer 1

The half life of K-40 is 1.3*10^9 years.
That is something they should have told you, or which you have to look up.

3.9*10^9 years (the approximate age of the earth) is three half lives.

100g*(1/2)^3 = ??

Answer 2

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