Radium-226 is a metal with a half-life of 1600 years. if you start with 1 kilogram of radium-226, how much will remain after 20,000 years?

Ending Amount = Beginning Amount / 2^n 
where 'n' = # of half lives
n = 20,000 / 1,600 = 12.5 half-lives

Ending Amount = 1 kg / 2^12.5
Ending Amount = 1 kg / 5,792.6
Ending Amount = 0.00017263 kilograms = 0.17263 grams