\documentclass[a4paper,12pt]{article}
\usepackage[draft]{probsoln}

\begin{document}
\begin{defproblem}[1][{2}]{diffsin}
Differentiate $f(x)=\sin(#1x)$.
\begin{onlysolution}%  
  \begin{solution}
    $f’(x) = #1\cos(#1x)$
  \end{solution}
\end{onlysolution}
\end{defproblem}

\begin{enumerate}
	\item \useproblem{diffsin}
	%Same thing with \foreachproblem{\item\thisproblem}
\end{enumerate}
\end{document}


