The half-life equation is written as:
An = Aoe^-kt
We use this equation for the solution. We do as follows:
0.5 = e^-k(1590)
k = 0.00044
An = 300oe^-0.0004(4000)
An = 52.46 mg will remain after 4000 years
Hope this answers the question. Have a nice day.