`._(19)^(49)K` isotope of potassium has a half-life of `1.4 xx10^(9)` yr and decays to form stable argon, `._(18)^(40)Ar`. A sample of rock has been taken which contains both potassium and argon in the ratio `1:7`, i.e.,
`("Number of potassium `-14` atoms")/("Number of argon`-40` atoms")=(1)/(2)`
Assuming that when the rock was fromed no argon `-40` was present in the sample and none has escaped subssequently, determine the age of the rock.

Answer

`4.2 xx 10^(9)` years