Emulate \hfill in LaTeX equations?

Question

I have a set of equations, where each line consists of a formula and an explanation (info about indices). Now, let's say the structure looks like this:

\newcommand{\lw}{\linewidth}
\begin{align*}
    &\framebox[0.4\lw]{Formula 1} & \framebox[0.3\lw]{Info 1}& \\
    &\framebox[0.7\lw]{Formula 1} & \framebox[0.1\lw]{Info 1}& \\
    &\framebox[0.5\lw]{Formula 1} & \framebox[0.2\lw]{Info 1}&
\end{align*}

This results in two columns of widths 0.7\lw and 0.3\lw, so the equation will be too wide for the page:

Yet, no single line is wider than 0.8\lw, so there is enough space if the two columns could "slide into each other", like this:

In essence, I need some equiv. of \hfill between the two parts - but \hfill does not work here...

My question is whether there is a way to do this, i.e. to create a flexible space that would keep the left and right aligns. Note that the first align would typically be somewhere inside the formula, not in front of it.

Answer 1

I wouldn't put the conditions flush right, but if you really insist, you can use flalign:

\begin{flalign*}
\qquad A&=\mathrlap{B} &\text{condition}\\
\qquad C&=\mathrlap{D} &\text{cond}\\
\qquad E&=\mathrlap{F} &\text{some longer condition}
\end{flalign*}

Change the \qquad into some other spacing command that suits your needs.

This needs the package mathtools