Would the common factor be 1?
edit
the least common is 1 and the most common is 2
thanks andreadiaz2003
This is a relatively simple half life calculation.
To go from 6 to 3 grams, 1622 years is required.
3 to 1.5 grams another 1622 years is required.
So, it undergoes exactly 2 half-lives or 3,244 years.
To see more about half-life click here:
http://www.1728.org/halflife.htm