How to put two equations in one line with different number?

Question

I have to short equations, and consume to much vertical space, I want to gather them in one line, but with different number.

I want to the equation show like below

a = b (1)  c = d (2)
d = e (3)  e = f (4)

Answer 1

In case you don't need alignment across the equations (e.g. aligning all the equal signs), then a table can be a quick solution:

\documentclass{article}

\usepackage{lipsum}

\begin{document}

\lipsum[2]

\begin{center}
\begin{tabular}{p{3cm}p{3cm}}
  \begin{equation}
    a = b
  \end{equation}
  &
  \begin{equation}
    c = d
  \end{equation} \\
  \begin{equation}
    d = e
  \end{equation}
  &
  \begin{equation}
    e = f
  \end{equation}
\end{tabular}
\end{center}

\lipsum[2]

\end{document}