A sample of radium weighs 1000g.How long will it take to decay to 125g if its half-life is 1620 years?

Answer 1

counting half lifes
1000/500/250/125 that is three half lives. or 1620x3 years.
Now, math 1 way/
125=1000((1/2)^halflives
taking logs of each side.
log 125=log 1000+halflives*log(1/2)
(log125-log1000)/log(1/2) = halflives
halflives = (2.07-3)/.301=3.08 halflives (note I rounded log 125)
When one uses halflives,rounding errors often matter.

Answer 2

Yes