How to link Questions to Answers - Hyperlink and hypertarget - Multiple chapters

Question

For a Question bank, which is a pdf-only CC edition, I defined hyperlinks and hypertargets as explained in Adding another answer hyperlinked to the question itself.

To typeset more chapters as mentioned in the comments of this answer I tried adding \thechapter\thesection\thequestion. I was successful in linking Problems to Shortsolutions and Longsolutions. But reverse linking is failing.

Can somebody help me, I am posting my MWE.

\documentclass{memoir}

\usepackage{answers}                            
\usepackage{enumitem}                       
\usepackage{hyperref}
\usepackage{multicol}

\usepackage{pgffor}                            




\hypersetup{colorlinks=true}

\setlength{\parindent}{0.0mm}


% very useful during de-bugging!
\usepackage[left]{showlabels}
\showlabels{hypertarget}
\showlabels{hyperlink}

% solutions files
% short solutions
\Newassociation{shortsolution}{shortSoln}{shortsolutions}
% long solutions
\Newassociation{longsolution}{longSoln}{longsolutions}

% new environment that sets up hypertargets both in the question
% section, and in the answer section
\newlist{myenum}{enumerate}{3}
\newcounter{question}[chapter]
\newenvironment{question}[1]%
    {%
    \refstepcounter{question}%
    %  hyperlink to solution
         \hypertarget{question:{\thequestion}}{}%
         \Writetofile{shortsolutions}{\protect\hypertarget{shortsoln:\thequestion}{}}%
         \Writetofile{longsolutions}{\protect\hypertarget{longsoln:\thequestion}{}}%
         \begin{myenum}[label=\bfseries\protect\hyperlink{shortsoln:\thequestion}{\thequestion},ref=\thequestion]
         \item%
            \hypersetup{linkcolor=blue}%
            \hyperlink{longsoln:\thequestion}{#1}%
        }%
        {%
        \end{myenum}}

\begin{document}

\chapter{Questions and Answers}


%\Opensolutionfile{ans}[\jobname ans\thechapter]


\Opensolutionfile{shortsolutions}[\jobname key\thechapter]
\Opensolutionfile{longsolutions}[\jobname sol\thechapter]

\section{Questions}
\begin{multicols}{3}
    \begin{question}{$\int\sqrt{4-x}\mathrm{d}x$}
            \begin{shortsolution}
                $-\frac{2}{3}(4-x)^{\frac{3}{2}}+C$
            \end{shortsolution}
            \begin{longsolution}
                In order to do this, we must first make a substitution\ldots
            \end{longsolution}
    \end{question}
    \begin{question}{$\int(2x-1)^{50}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{1}{102}(2x-1)^{51}+C$
            \end{shortsolution}
            \begin{longsolution}
                A very long solution| much longer than the shorter solution
            \end{longsolution}
    \end{question}
    \begin{question}{$\int\sqrt[n]{x}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{n}{n+1}x^{\frac{1+n}{n}}+C$
            \end{shortsolution}
            \begin{longsolution}
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
            \end{longsolution}
    \end{question}
\end{multicols}

% close solution files
\Closesolutionfile{shortsolutions}
\Closesolutionfile{longsolutions}






\chapter{Two and Second Similar One}



%\Opensolutionfile{ans}[\jobname ans\thechapter]


\Opensolutionfile{shortsolutions}[\jobname key\thechapter]
\Opensolutionfile{longsolutions}[\jobname sol\thechapter]

\section{Questions}
\begin{multicols}{3}
    \begin{question}{$\int\sqrt{4-x}\mathrm{d}x$}
            \begin{shortsolution}
                $-\frac{2}{3}(4-x)^{\frac{3}{2}}+C$
            \end{shortsolution}
            \begin{longsolution}
                In order to do this, we must first make a substitution\ldots
            \end{longsolution}
    \end{question}
    \begin{question}{$\int(2x-1)^{50}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{1}{102}(2x-1)^{51}+C$
            \end{shortsolution}
            \begin{longsolution}
                A very long solution| much longer than the shorter solution
            \end{longsolution}
    \end{question}
    \begin{question}{$\int\sqrt[n]{x}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{n}{n+1}x^{\frac{1+n}{n}}+C$
            \end{shortsolution}
            \begin{longsolution}
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
            \end{longsolution}
    \end{question}
\end{multicols}

% close solution files
\Closesolutionfile{shortsolutions}
\Closesolutionfile{longsolutions}


\chapter{Answers to the Problems}
% renew the SHORT solution environment so that it hyperlinks back to 
% the question
\renewenvironment{shortSoln}[1]{%
         % add some glue
         \vskip .5cm plus 2cm minus 0.1cm%
         {\bfseries \hyperlink{question:#1}{#1.}}%
}%
{%
}%
% input the file if it exists
%\IfFileExists{shortsolutions.tex}{\input{shortsolutions.tex}}{}




 \foreach \i in {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...,15} {%
    \edef\FileName{\jobname key\i}%     The % here are necessary to eliminate any
    \IfFileExists{\FileName}{%  spurious spaces that may get inserted
        \input{\FileName}%       at these points
    }
 }






\clearpage
\chapter{Solutions}
% renew the LONG solution environment so that it hyperlinks back to 
% the question
\renewenvironment{longSoln}[1]{%
         % add some glue
         \vskip .5cm plus 2cm minus 0.1cm%
         {\bfseries \hypersetup{linkcolor=blue}\hyperlink{question:#1}{#1.}}%
}%
{%
}%
% input the file if it exists
%\IfFileExists{longsolutions.tex}{\input{longsolutions.tex}}{}

%Input all Solutions files as they appear Chapter-wise properly hyperlinked. 

 \foreach \i in {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...,15} {%
    \edef\FileName{\jobname sol\i}%     The % here are necessary to eliminate any
    \IfFileExists{\FileName}{%  spurious spaces that may get inserted
        \input{\FileName}%       at these points
    }
 }


\end{document}