Solve the system by substitution. \[ \begin{aligned} x & =9 y \\ x-5 y & =20\end{aligned} \]

First, we can substitute the expression for \( x \) from the first equation into the second equation. Since \( x = 9y \), we rewrite the second equation as: \[ 9y - 5y = 20 \] Now, combine like terms: \[ 4y = 20 \] Next, divide both sides by 4 to find \( y \): \[ y = 5 \] Now that we have \( y \), we can substitute this value back into the first equation to find \( x \): \[ x = 9y = 9(5) = 45 \] Thus, the solution to the system is: \[ (x, y) = (45, 5) \]