Answer
(a) See explanations.
(b) $30$ g/L
Work Step by Step
(a) At time $t$ minutes, there will be $V_1=25t$ L of brine in the solution containing $W_1=30V_1$ g of salt.
The total volume will be $V=V_1+5000$ and the concentration can be calculated as:
$C(t)=\frac{W_1}{V}=\frac{30V_1}{V_1+5000}=\frac{30\times25t}{25t+5000}=\frac{30t}{t+200}$ g/L
(b) Rewrite the above equation as $C(t)=\frac{30}{1+200/t}$ and as $t\to\infty$, we have
$$\lim_{t\to\infty}\frac{30}{1+200/t}=30$$ or $C(\infty)=30$ g/L
