How can change an equation number according to where it is mentioned for the first time?

Question

I am writing 2 long equations in the top of a page in a paper. For paper construction and presentation purposes, I want to write the both in one place (so their code are brought together using Figure environment). The problem is that the second one will be introduced one page later and I want its number adjusted according to the place I mention it for the first time, not the place of it in the code. Is this possible? (For clarification, first equation number is 35, then I have to bring 5 equations to obtain the second formula whose number should be 41, but now it is 36 and those 5 equations required to obtain second one is numbered after that.) When I use \tag, latex automatically assign it again to another equation later.

Answer 1

If you want the equation number to come from a specific location, you can use \refstepcounter{equation}\label{...} at the location and \tag{\ref{...}} in the float.

\documentclass{article}
\usepackage{amsmath}
\usepackage{lipsum}% MWE only

\begin{document}
\begin{figure}[b]% just to force a delay
\begin{equation}
 x=b \tag{\ref{mary}}
\end{equation}
\end{figure}

Given
\begin{equation}
  x=a
\end{equation}
and
\begin{equation}
  a=b
\end{equation}
we can obtain the equation on the bottom of the page.%
\refstepcounter{equation}\label{mary}

\lipsum[1-8]
\end{document}