if you start with a 1kg sample after 1620 years there will be 0.5kg
after 3240 years (another 1620) there will be 0.25kg
after 4860 years (another 1620) there will be 0.125kg of radium left.
There is a kinda complicated formula for things that aren't in neat little divisions like that but i cant remember it. I can find it if you want but i dont know if you will need to know it.
