# Dot separate align numbering

When you're numbering equations steps with align is it possible to separate the problem numbering like (problem #).(step) like they did here?

• How is your problem environment defined? – Werner Nov 27 '18 at 3:58

Straight from the amsmath documentation (PDF):

\documentclass{article}
\usepackage{amsmath}{document}

\begin{subequations}
\renewcommand{\theequation}{\theparentequation.\arabic{equation}}
\begin{align}
a &= b \\
c &= d
\end{align}
\end{subequations}

\end{document}