An isotope of Potassium `._(19)^(40) K ` has a half life of `1.4 xx 10^(9)`year and decays to Argon `._(18)^(40)Ar`which is stable.
(i) Write down the nuclear reaction representing this decay.
(ii) A sample of rock taken from the moon contains both potassium and argon in the ratio `1//3`. find age of rock