ah, yes
if you start with P amount of something and the half life is h units (in years) and the time elapsed is t (in years) then the amount left is A
the equation is
[tex]A=P(\frac{1}{2})^\frac{t}{h}[/tex]
so
given
h=1620
P=150
t=200
so
[tex]A=150(\frac{1}{2})^\frac{200}{1620}[/tex]
[tex]A=150(\frac{1}{2})^\frac{10}{81}[/tex]
use your calculator
A≈137.69781917515g
about 138grams left
