⁴⁰K Decays to ⁴⁰Ar with a Half-Life of 1

Multiple Choice

⁴⁰K decays to ⁴⁰Ar with a half-life of 1.25 x 10⁹ yr.Assume that rocks contain no ⁴⁰Ar when they form, and that the only way ⁴⁰Ar can be present is through the decay of ⁴⁰K.If the ratio of ⁴⁰K to ⁴⁰Ar in a particular rock is found to be 1:3, what is the age of the rock?

A) 1.25 x 10⁹ yr
B) 2.50 x 10⁹ yr
C) 3.75 x 10⁹ yr
D) 5.00 x 10⁹ yr
E) cannot be determined without knowing how much ⁴⁰K was in the rock to begin with