\documentclass{article}
\usepackage{proof}
\pagestyle{empty}
\textwidth18cm
\textheight26cm
\oddsidemargin-1cm
\evensidemargin\oddsidemargin
\headheight0cm
\headsep0cm
\footskip0cm
\topmargin0cm
\parindent0pt

\newenvironment{problem}{\begin{description}\item[Problem:]}{\end{description}}
\newenvironment{answer}{\begin{description}\item[Answer:]}{\end{description}}
\begin{document}
