You can
put this solution on YOUR website!Your formula certainly works and your answer is essentially
correct. I use a different formula for half-life problems.
For what it is worth I get:
0.59A = A(1/2)^(t/5730)
0.59 = (1/2)^(t/5730)
ln0.59 = (t/5730)(ln0.5)
t=5730[(ln0.59/ln0.5]
t= 4361.75 yrs.
Cheers,
Stan H.