smc

Our first procedure is to find the value of $g$. With the given variables' values, we can see that $8g-4=16$ so $g=\frac{20}{8}=\frac{5}{2}$. With that, we can replace $g$ with $\frac{5}{2}$. When $S=10$, we can see that $10\times \frac{5}{2}-4=\frac{50}{2}-4=25-4=\boxed{21}$.