We see that in 1 rock, there are 31 atoms of Argon and 1
atom of Potassium so the relative concentration of Potassium is:
1 / 32
or can be written as:
1 / 2^5
So this means that 5 half-lives have passed.
So the years are:
years passed = 5 * 1.25 billion years = 6.25 billion years
