This is quite similar to a writing questions and answers in separate places; I'm a huge fan of the answers package, which is what I've used in the solution below.
Stepping the counter is a little clumsy (perhaps someone will have suggestions for improvements), but I think it does what you want.
\documentclass{article}
\usepackage{lipsum} % for sample text
\usepackage{amsmath} % for mathematical environments
\usepackage{answers} % for file writing
\usepackage{etoolbox} % for \AtBeginEnvironment (and a lot more)
\usepackage{hyperref} % for hyperlinks- load it last
% hyperlinks
\hypersetup{colorlinks=true,
linkcolor=blue}
% very useful during de-bugging!
%\usepackage[left]{showlabels}
%\showlabels{hypertarget}
%\showlabels{hyperlink}
% appendix file
\Opensolutionfile{appendixmaterial}
\Newassociation{appmat}{appMAT}{appendixmaterial}
% setup hypertargets for equations
\AtBeginEnvironment{equation}{%
\stepcounter{equation}%
\hypertarget{q:myref\theequation}{}%
\Writetofile{appendixmaterial}{\protect\hypertarget{a:myref\theequation}{}}%
\Writetofile{appendixmaterial}{\protect\label{a:myref\theequation}{}}%
\addtocounter{equation}{-1}%
}
% command to link to the appendix
\newcommand{\linktoappmat}{%
details of equation \hyperlink{a:myref\theequation}{\theequation} (see page \pageref{a:myref\theequation})}
\begin{document}
\lipsum[1]
\begin{equation}
2\cdot (3+3)=12
\begin{appmat}
\begin{align*}
2\cdot (3+3) & =2\cdot 3+2\cdot 3 \\
&=6+6 \\
& =12 \\
\end{align*}
\end{appmat}
\end{equation}
\linktoappmat
\begin{equation}
2\cdot (5+5)=12
\begin{appmat}
\begin{align*}
2\cdot (5+5) & =2\cdot 5+2\cdot 5 \\
&=10+10 \\
& =20 \\
\end{align*}
\end{appmat}
\end{equation}
\linktoappmat
% close solution file
\Closesolutionfile{appendixmaterial}
% renew the solution environment so that it hyperlinks back to
% the question
\renewenvironment{appMAT}[1]{%
% add some glue
\vskip .5cm plus 2cm minus 0.1cm%
{details of equation \bfseries \hyperlink{q:myref#1}{#1.}}%
}%
{%
}%
\clearpage
\section{Appendix}
% input the file if it exists
\IfFileExists{appendixmaterial.tex}{\input{appendixmaterial.tex}}{}
\end{document}
For another example of this kind of thing, have a look at Adding another answer hyperlinked to the question itself and the link within.