Answer
$400 \ gm$
Work Step by Step
Suppose that $D_n$ be the amount of drug available in the blood after every six hours.
So, we can write as: $D_1= (\dfrac{1}{2})(200)+200=300 \\ D_2= (\dfrac{1}{2})(300)+200=350\\D_3= (\dfrac{1}{2})(350)+200=375\\D_n= (\dfrac{1}{2})(D_{n-1})+2000$
Next, $\lim\limits_{n \to \infty} D_{n-1}=C$
or, $C=0.5 C+200$
or, $C=400 \ gm$
