referencing one equation in system of equations latex
\documentclass{article}
\usepackage{amsmath}
\begin{document}
See the equations below:
\begin{subequations}
\label{all1}
\begin{align}
a & = b \label{eq1} \\
c & = d \label{e2}
\end{align}
\end{subequations}
These refer to \ref{all1}, \ref{eq1} and \ref{e2}.
\end{document}