Using both exam.cls and probsoln.sty
This example requires two LaTeX runs if you want to reference the number (or total number) of points.
\documentclass[addpoints]{exam}
\usepackage{probsoln}
\showanswers
\renewenvironment{solution}%
{\par\noindent\textbf{\solutionname}: \ignorespaces}%
{\ignorespacesafterend}
\begin{document}
\begin{center}\bfseries
Assignment~1\ifshowanswers\space (Solution Sheet)\fi
\end{center}
\begin{questions}
\question[1] What is the main drawback of ray guns?
\begin{onlysolution}
\begin{solution}
Overheating.
\end{solution}
\end{onlysolution}
\end{questions}
\end{document}
Download exam-probsoln.tex or exam-probsoln.pdf.