Radium 226, which had been used for cancer treatment, decays with a half-life of 1622 years. suppose there is a 6 gram sample of radium 226. how long will it take to have 1.5 grams remaining?

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