Question

The half-life of radium is 1690 years. If 50 grams are present now, how much will be present in 390 years?

Answer 1

\[ m_0 2^{-\frac{t}{1690}} \]

Answer 2

Ending Amount = Beginning Amount / 2^n
where n is elapsed time / half-life
Ending Amount = 50 / 2^(390/1690)
Ending Amount = 50 / 2^0.2307692308
Ending Amount = 50 / 1.17346046
Ending Amount = 42.6090198214 grams
or about 42.61 grams
a good web page about half life is
http://1728.org/halflife.htm
has formulas and a calculator