Column Width with Tabu Package

Question

How can I set particular column widths in tabu? Here's what I have at the moment

{\tabulinesep=1.2mm
\begin{tabu}{|X|X|X|X|}
    \hline
        \textbf{Question} & \textbf{Solution} & \textbf{Partial Marks} & \textbf{Guidance} \\
    \hline
        %%%%%%%%%%%%%%% Q1 Equation of a Line
        1a & $\frac{1}{2}$ & A1 & cao \\
    \hline
        1b & $y-\frac{1}{3}x+4=0$ & A2 & cao \\
    \hline
        %%%%%%%%%%%%%%% Q2 Indices
        2 & $\frac{3^{2x}}{3^{2y}}=3^{2x+4}$ & M1 & Converts $9^x$ to $3^{kx}$ where $k$ is an integer \\
    \hline

which gives me this

I want to fix the size of the left-hand column and third column smaller than they currently are, while ideally leaving the other two alone. Any advice would be appreciated, thanks.

Answer 1

As @HenriMenke has already pointed out in a comment, don't use the tabu package: it's buggy and it's unmaintained. Instead, I would suggest that you use the tabularx package and its eponymous environment. I would also recommend that you give the tabular material a much more open "look", mainly by omitting all vertical lines and using fewer, but well-spaced horizontal lines.

The following solution assumes that the overall width of the tabular material should be \textwidth. Since line-breaks don't seem to be useful or permissible in the first three columns, they use the l column type. Only the final column uses the X column type.

A final comment on the design of the table: Using boldface for the material in the header cells gives the table a rather "hefty" look -- but without actually improving its intelligibility and readability. If this were my table, I wouldn't boldface the words in the header row.

\documentclass{article} 
\usepackage{tabularx} % for 'tabularx' environment
\usepackage{booktabs} % for \toprule, \midrule, \bottomrule, and \addlinespace macros
\begin{document} 
\noindent
\begin{tabularx}{\textwidth}{@{} lll X @{}}
    \toprule
    \textbf{Question} & \textbf{Solution} & 
    \textbf{Partial Marks} & \textbf{Guidance} \\
    \midrule
    %%%%%%%%%%%%%%% Q1 Equation of a Line
    1a & $\frac{1}{2}$ & A1 & cao \\
    \addlinespace
    1b & $y-\frac{1}{3}x+4=0$ & A2 & cao \\
    \addlinespace
    %%%%%%%%%%%%%%% Q2 Indices
    2 & $\frac{3^{2x}}{3^{2y}}=3^{2x+4}$ & M1 & 
    Converts $9^x$ to $3^{kx}$ where $k$ is an integer \\
    \bottomrule
\end{tabularx}
\end{document}

Answer 2

The new LaTeX3 package tabularray is an alternative to the outdated tabu package. And Q[2cm] sets the column width.

\documentclass[a4paper,12pt]{report}

\usepackage{tabularray}

\begin{document}    

\noindent
\begin{tblr}{colspec={Q[2cm]XQ[2cm]X},row{1}={font=\bfseries},hlines}
 Question & Solution                         & Partial Marks & Guidance \\
 1a       & $\frac{1}{2}$                    & A1 & cao \\
 1b       & $y-\frac{1}{3}x+4=0$             & A2 & cao \\
 2        & $\frac{3^{2x}}{3^{2y}}=3^{2x+4}$ & M1 & Converts $9^x$ to $3^{kx}$ where $k$ is an integer \\
\end{tblr}

\end{document}

Answer 3

A small variant of @L.J.R. answer (for exercise):

\documentclass[a4paper,12pt]{report}
\usepackage{amsmath}
\usepackage{array,tabularray}
\NewColumnType{C}{>{$\displaystyle}X<{$}}

\begin{document}

\noindent
\begin{tblr}{colspec={Q[2cm] C Q[2cm,c,m] X[m]},
             row{1}={font=\bfseries},
             rowsep=3pt,
             hlines}
 Question & \text{Solution}                & Partial Marks  & Guidance \\
 1a       & \frac{1}{2}                    & A1             & cao \\
 1b       & y-\frac{1}{3}x+4=0             & A2             & cao \\
 2        & \frac{3^{2x}}{3^{2y}}=3^{2x+4} & M1             & Converts $9^x$ to $3^{kx}$ where $k$ is an integer \\
\end{tblr}

\end{document}