How to make \begin{align} looks like \begin{eqnarray}

Question

I trying to align a programming model using eqnarray but the command \tag do not work with it, I could use align instead but the alignment is right for the first column and left for the second column.

So what I want to do is use the eqnarray alignment (which is right for the first column and centered for the second column) with align.

This is how both commands look like:

Answer 1

Here is a solution using \mathmakebox from the mathtools package:

\documentclass{article}
\usepackage{mathtools}% loads `amsmath'
\DeclareMathOperator*{\Max}{Max}
\begin{document}
A solution with \verb|\mathmakebox|:
\begin{alignat}{2}
           \Max_t & \quad & F(t) = at, \\
\text{subject to} &       & \mathmakebox[\widthof{$F(t) = at,$}][c]{t \le b,} \\
                  &       & \mathmakebox[\widthof{$F(t) = at,$}][c]{t \ge 0.}
\end{alignat}
\end{document}

Answer 2

Here are two possibilities with alignat and eqparbox:

\documentclass{article}
\usepackage{mathtools}
\usepackage{eqparbox}
\newcommand{\eqmathbox}[2][M]{\eqmakebox[#1]{$\displaystyle#2$}}

\begin{document}

\begin{alignat}{2}
  \max_t & &\quad & \eqmathbox{F(t) = at}\\
 \text{subject to}& &
 & \eqmathbox{\begin{gathered}[t]
 t\le b, \\t\ge 0.
 \end{gathered}}
\end{alignat}
\begin{alignat}{2}
  \max_t & &\quad & \eqmathbox{F(t) = at}\\
 \text{subject to}& &
 & \eqmathbox{t \le b,} \\
  & & & \eqmathbox{t \ge 0.}
\end{alignat}

\end{document}