\begin{exercises}[columns=2]
\begin{exercise}
Calculate $y = 5 + 7$
\end{exercises}
\begin{solution}
$y = 12$
\end{solution}
\begin{exercise}
Calculate $y = 7 - 12$
\end{exercises}
\begin{solution}
$y = -5$
\end{solution}
\end{exercises}

\begin{exerciseseries}[columns=2,subrule=\hrule]{Easy exercises}
\begin{exercise}
Calculate $y = 5 + 7$
\end{exercise}
\begin{solution}
$y = 12$
\end{solution}
\begin{exercise}
Calculate $y = 7 - 12$
\end{exercise}
\begin{solution}
$y = -5$
\end{solution}
\end{exerciseseries}
\begin{exerciseseries}{Difficult exercises}
\begin{exercise}
Calculate $y = 5 \cdot 7$
\end{exercise}
\begin{solution}
$y = 35$
\end{solution}
\begin{exercise}
Calculate $y = 8 / 4$
\end{exercise}
\begin{solution}
$y = 2$
\end{solution}
\end{exerciseseries}