# Multicolumn width calculated for lX columns

I have the following table:

\documentclass{article}
\usepackage{tabu, longtable}

\begin{document}
\begin{longtabu} to \textwidth {|l|l|l|lX|}

1 & Column 2 & 3 & \multicolumn{2}{l|}{Description} \\ \hline
1 & 2 & 3 & A: & Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer eget est id enim vehicula sodales et quis ipsum. \\ \hline

& & & \multicolumn{2}{p{8cm}|}{Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer eget est id enim vehicula sodales et quis ipsum.} \\ \hline

\end{longtabu}
\end{document}


The last row uses a fixed width for the paragraph (8cm). This does not fit perfectly but shows how the result should look.

My question is: Is it possible to replace the fixed value by something else so Latex calculates the correct width of the last two columns for me?

-
This is not a duplicate because here the column types are l and X while in the mentioned question both are X. Therefore 2\tabucolX won't work here. –  Severin Oct 10 '12 at 14:59
oops sorry I am sure that I have done the general case somewhere on site I think I picked the wrong question. I don't seem to be able to cancel my close vote. tabularx (and I presume tabu) does know all the column widths as it has to disasemble the table after each trial, it's just a bit less convenient to get the other widths, will post later. –  David Carlisle Oct 10 '12 at 15:19

Actually what I said in comments wasn't quite right tabularx only really knows the widths of the X columns and the total table width. longtable on the other hand does know (and record in the aux file) all the column widths, tabu is using a mixture of the two so we can use the longtable data:

This could be wrapped up in a nicer interface but basically you need to do:

\documentclass{article}
\usepackage{tabu, longtable}

\begin{document}
\makeatletter
\newdimen\zzz
\ifx\LT@i\@undefined
\zzz=8cm
\else
\global\zzz\z@
\begingroup
\@tempcnta\z@
\def\LT@entry#1#2{%
}
\LT@i
\endgroup