For any real numbers $a$ and $b,$ if $a=b,$ then $a$ may be replaced by $b$ in any expression containing $a$ and vice versa. This property is one of the Properties of Equality that can be used when solving equations. $$\begin{array}{c}\{\begin{array}{l}2x+1=20\\ y(2x+1)=40\end{array}\Rightarrow \begin{array}{rl}y(20)& =40\\ y& =2\end{array}\end{array}$$ Because $2x+1$ is equal to $20,$ it can be replaced into the second equation to obtain the value of $y.$ Keep in mind that the Substitution Property of Equality also holds true if $a$ and $b$ are complex numbers.