1/2 life of R 226 if A(t)=600e^-0.00043t
i got stuck
A(t)=A(t)=600e
A(t)=600/2=300
300=A(t)=600e
300 = 600e^-.00043t
1/2 = e^-.00043t
-ln2 = -.00043t
t = 1612
so, after 1612 years, half is gone. That's the half-life.
Check: web search reveals a half-life of 1600 years or so.