\documentclass{article}
\begin{document}
The current value of \verb|\textwidth| is: \the\textwidth.
\advance \textwidth by 55pt
If we add 55pt to \verb|\textwidth|, the value of \verb|\textwidth| becomes \the\textwidth, 
\advance \textwidth by -90pt
and if we subtract 90 pt from \verb|\textwidth|, the value of \verb|\textwidth| becomes \the\textwidth.
\end{document}