The half-life of radium is 1690 years. If 20 grams are present now how much will be present in 960 years

Answer 1

1/2 = e^1690r

ln.5 = 1690r

r = (ln.5)/1690

A = 20e^960(ln.5)/1690

= 20e^(96/169)ln.5

= about 13.49 grams left after 960 years

960 is over half of 1690, so there should be more 1/2 of 20 left, more than 10 grams