A 100-liter tank contains saltwater with a concentration of 10 g/L. Fresh water flows in at 5 L/min while the mixed solution drains at 5 L/min (volume constant). The salt amount $S(t)$ satisfies $\frac{dS}{dt} = -\frac{S(t) \cdot 5}{100}$. Approximately how many grams of salt remain after 30 minutes?