G = 40n + 20k, for k
\(G=40n+20k\ \ \ \ |Subtract\ 40n\\\\ G-40n=20k\ \ \ |Divide\ by\ 20\\\\ \frac{G-40n}{20}=k\\\\ k=\frac{G}{20}-2n\\\\Solution\ is\ k=\frac{G}{20}-2n.\)