Question

the half life of radium is 1690 years. if 100 grams are present now, when will there be 20% of the original amount​

Answer 1

Let A=the remaining amount of radium
Let S=the amount of radium you start with
Let t=the amount of time in years
Let h=the half-life time

Then A=S(1/2)^(t/h)
So,
A=80(1/2)^(430/1690)
A=approx. 67g