A lunar rock was analyzed for argon by mass spectrometry and for potassium by atomic absorption. The results of these analyses showed that the sample contained 3.02 x 10^-5 mL g^-1 at STP of argon and 0.083% of potassium. The half-life of potassium-40 is 1.26 x 10⁹ years. Calculate the age of the lunar rock.