Hyperlinks from exercises to solutions and vice versa

Question

Writing lecture notes containing exercises and solutions, I would like to create hyperlinks from the exercises to their solutions and vice versa. I use the packages mdframed, hyperref and answers (and for some reasons I would like to stick at least with the first two). The example below almost does what I want. However, the hyperlink from an exercise to its solution is now the name ("Exercise") of the exercise, while I would like it to be its number ("1"). How can I achieve this?

\documentclass{book}
\usepackage{answers}
\usepackage{hyperref}
\usepackage[framemethod=tikz]{mdframed}
\Newassociation{sol}{answ}{ans}\Opensolutionfile{ans}
\mdtheorem{ex}{\hyperlink{ex:\theex}{Exercise}\hypertarget{sol:\theex}{}}
\renewenvironment{answ}[1]{{\hypertarget{ex:#1}{}\hyperlink{sol:#1}{#1}}\quad}{}
\begin{document}
\begin{ex}
This is an exercise.
\begin{sol}
This is the solution.
\end{sol}
\end{ex}
\Closesolutionfile{ans}
\input{ans}
\end{document}

Answer 1

Although you want to stick to mdframed I propose a solution with tcolorbox because it can provide mdframed+answer behaviour. This solution also uses hyperref for links.

Like answers, tcolorbox offers some commands and environments to define boxes with parts which will be saved on an external file and printed later on.

Following code is a simplification of example in section 8.3 from tcolorbox documentation (something similar can be seen here)

Every ex box will save the solution (text after \tcblower) into an independent file.

Command \tcbstartrecording[file1] will start to save into file1 the name of all exercises to be included into the solutions list.

Command \tcbstoprecording does what it says.

Command \tcbinputrecords[file1] loads all solutions and print them.

About references, I've suposed that every exercise contains a solution and they respective names are references to their solution/exercise.

\documentclass{article}
\usepackage[most]{tcolorbox}
\usepackage{hyperref}
\NewTColorBox[auto counter]{ex}{+O{}}{%
        enhanced, colframe=black, colback=white, sharp corners, 
        attach title to upper=\par, coltitle=black, fonttitle=\bfseries,
        title={Exercise~\ref{solution@\thetcbcounter}},
        label={exercise@\thetcbcounter},
        lowerbox=ignored,
        savelowerto=solutions/exercise-\thetcbcounter.tex,
        record={\string\solution{\thetcbcounter}%
               {solutions/exercise-\thetcbcounter.tex}},
        #1}
\NewTotalTColorBox[auto counter]{\solution}{mm}{%
        enhanced, blanker,

        coltitle=black, fonttitle=\bfseries,
        title={\ref{exercise@\thetcbcounter}},
        label={solution@\thetcbcounter},
        attach title to upper={\ },
        }{\input{#2}}
\begin{document}
% We start to record all tcblower parts in file file1 for later processing 
\tcbstartrecording[file1]
%Every exercise box contains an upper part which is processed and
%printed and a lower (solution) which printing is delayed.
%
\begin{ex}
This is an exercise.
\tcblower
This is the solution for first exercise
\end{ex}
\begin{ex}
This is an exercise.
\tcblower
This is the solution for second exercise
\end{ex}
\tcbstoprecording
%previous recorded files with solutions are processed and printed
\tcbinputrecords[file1]
\end{document}

Answer 2

You can redefine \theex at the beginning of the mdtheorem, using startcode option:

\documentclass{book}
\usepackage{answers}
\usepackage{hyperref}
\usepackage[framemethod=tikz]{mdframed}
\Newassociation{sol}{answ}{ans}\Opensolutionfile{ans}
\mdtheorem[startcode={\let\theexold\theex\renewcommand{\theex}{\hyperlink{ex:\theexold}{\theexold}\hypertarget{sol:\theexold}{}}}]{ex}{Exercise}
\renewenvironment{answ}[1]{{\hypertarget{ex:#1}{}\hyperlink{sol:#1}{#1}}\quad}{}
\begin{document}
\begin{ex}
    This is an exercise.
    \begin{sol}
        This is the solution.
    \end{sol}
\end{ex}

\begin{ex}
    This is an exercise.
    \begin{sol}
        This is the solution.
    \end{sol}
\end{ex}
\Closesolutionfile{ans}
\input{ans}


\end{document}

Answer 3

Another proposal using xsim instead of answers but using your requirement mdframed:

\documentclass{book}
\usepackage[framemethod=tikz]{mdframed}
\usepackage{xsim}
\usepackage{hyperref}
\DeclareExerciseEnvironmentTemplate{myex}{%
  \begin{mdframed}[%
    frametitle={%
      \hypertarget{ex:\ExerciseID}{\XSIMmixedcase{\GetExerciseName}}
      \hyperlink{sol:\ExerciseID}{\GetExerciseProperty{counter}}%
    }]%
}{\end{mdframed}}
\DeclareExerciseEnvironmentTemplate{mysol}{%
  \par\noindent
  \hyperlink
    {ex:\ExerciseID}
    {\hypertarget{sol:\ExerciseID}{\GetExerciseProperty{counter}}}%
  \quad
}{\par}
\xsimsetup{
  exercise/template = myex ,
  solution/template = mysol ,
  print-solutions/headings = false
}
\begin{document}
\begin{exercise}
  This is an exercise.
\end{exercise}
\begin{solution}
  This is the solution.
\end{solution}
\begin{exercise}
  This is another exercise.
\end{exercise}
\begin{solution}
  This is the other solution.
\end{solution}
\printsolutions
\end{document}

Should you later want to switch to tcolorbox instead it would only require changing the template definition myex:

\DeclareExerciseEnvironmentTemplate{myex}{%
  \begin{tcolorbox}[sharp corners, colback = white]
    \textbf{%
      \hypertarget{ex:\ExerciseID}{\XSIMmixedcase{\GetExerciseName}}
      \hyperlink{sol:\ExerciseID}{\GetExerciseProperty{counter}}%
    }\par\smallskip
}{\end{tcolorbox}}

It would also be possible to define the environments ex and sol if you'd insist on these names:

Load the package with the blank option, it now does not define exercise and solution

\usepackage[blank]{xsim}

and define ex and sol:

\DeclareExerciseType {exercise} {
  exercise-env      = ex ,
  solution-env      = sol ,
  exercise-name     = \XSIMtranslate {exercise} ,
  exercises-name    = \XSIMtranslate {exercises} ,
  solution-name     = \XSIMtranslate {solution} ,
  solutions-name    = \XSIMtranslate {solutions} ,
  exercise-template = myex ,
  solution-template = mysol
}

As option setup now only \xsimsetup{print-solutions/headings = false} is needed in the above example and the environmants can be changed:

\begin{ex}
  This is an exercise.
\end{ex}
\begin{sol}
  This is the solution.
\end{sol}
\begin{ex}
  This is another exercise.
\end{ex}
\begin{sol}
  This is the other solution.
\end{sol}