well count how many half lifes it would take to get from 1 to 0.125 first
1 to 0.5 - 1 half life
0.5 to 0.25 - 2nd half life
0.25 to 0.125 - 3rd half life
so essentially it would take 3 half lives to get to 0.125 kg of radium.
Now simply multiply the number of half lives with the half life value given
(3x1620) to get your answer as the number of years that it would take.
Hope this helps
