Some tweaking might be needed to suit your needs, but I think what follows may be of help. I've defined a "two-part" environment called qanda (for "Question and Answer"), split by a command called \nextpart, for each couple of question and answer. You can finetune the space allocated to each answer by using \mbox{}\vspace{...} after \nextpart.

\documentclass[12pt,landscape]{article}
\usepackage[showframe,a4paper]{geometry} % change to a3paper if needed
\usepackage{etoolbox}
\usepackage{enumitem}
\newtoggle{morethanoneparagraph}
\togglefalse{morethanoneparagraph}
\newcommand{\mycolumnwidth}{0.45\textwidth}
\newenvironment{qanda}{% Q&A
\newcommand\nextpart{%
\end{enumerate}
\end{minipage}
\begin{minipage}[t]{\mycolumnwidth}
\iftoggle{morethanoneparagraph}{%
\begin{enumerate}[label=\roman*.,resume*=q]
\item
}{%
\global\toggletrue{morethanoneparagraph}
\begin{enumerate}[label=\roman*.,series=q]
\item
}
}
\begin{minipage}[t]{\mycolumnwidth}
\iftoggle{morethanoneparagraph}{
\begin{enumerate}[label=\arabic*.,resume*=a]
\item
}{
\begin{enumerate}[label=\arabic*.,series=a]
\item
}
}{
\end{enumerate}
\end{minipage}
\vspace{\baselineskip}
}
\setlength{\parindent}{0pt}
\begin{document}
\begin{qanda}
Prove or disprove $P=NP$
\nextpart \mbox{}\vspace{5cm}
\end{qanda}
\begin{qanda}
Solve
\[
x^2-x+1=0\,.
\]
\nextpart
\mbox{}\vspace{6cm}
\end{qanda}
\begin{qanda}
Disprove Fermat's Last Theorem.
\nextpart
``Mrs Wiles\ldots please get your husband on the phone\ldots''
\end{qanda}
\end{document}
exsheets. If a solution using it instead ofexamwould be welcome I'll post an answer. – cgnieder Mar 09 '13 at 18:55