documentclass[12pt,a4paper]{book}
usepackage{answers}
usepackage{amsthm}
usepackage{hyperref}
usepackage{tcolorbox}
usepackage{ifthen}
usepackage{tikz}
usetikzlibrary{shadows}

tikzstyle{buttonstyle} = [rectangle, fill = black!30, draw = black!80, drop shadow, font={sffamilybfseries}, text=white]

newcommand*{button}[1]{tikz{node[buttonstyle] {#1};}}

theoremstyle{definition}
newtheorem{ex}{%
  hypertarget{sol:theex}{ } hfill
  hyperlink{ex:theex}{button{Solution}}\
  phantomsection
  }[chapter]
Newassociation{sol}{Soln}{corr}
renewenvironment{Soln}[1]
  {parbigskipnoindentphantomsection{bfseries hypertarget{ex:#1}{}
{bfseries Solution  of exercise #1}hfill
  hyperlink{sol:#1}{button{Back to exercise #1}}\}quad}
  {parbigskip}

% Usage MEExercise[keys]{exercise}{solution}
newcommandMEExercise[3][]{
ifthenelse{equal{#3}{}}
{ % The no solution case
}
{ %Have a solution
begin{tcolorbox}
begin{ex}
#2
begin{sol}
#3
end{sol}
end{ex}
end{tcolorbox}
}
}

begin{document}
chapter{This}
Opensolutionfile{corr}[ans1]
section{Problems}
begin{tcolorbox}
begin{ex}
First exercise.
begin{sol}
First solution.
end{sol}
end{ex}
end{tcolorbox}

%MEExercise{this is exercise 1}{this is solution of exercise 1}
chapter{That}
%newpage
begin{ex}
Second exercise.
begin{sol}
Second solution.
end{sol}
end{ex}
%MEExercise{this is exercise 2}{this is solution of exercise 2}
%newpage
Closesolutionfile{corr}
%newpage
section{Solutions}
input{ans1}
end{document}

begin{Soln}{1.1}
First solution.
end{Soln}
begin{Soln}{1.2}
this is solution of exercise 1 end {sol} end {ex} end {tcolorbox}
chapter{That}
%newpage
begin{ex}
Second exercise.
begin{sol}
Second solution.
end{Soln}