\documentclass{book}
\usepackage{lipsum,adjmulticol}
\usepackage{color}
%%%%%%
\usepackage{newfile}
\usepackage{xifthen}
\usepackage{enumitem}
\usepackage{amsmath}
%%%%%%%%%%%%%%%
\usepackage{ptext}
%%%%%%%%%%%%%%%%%%%%% دستورات مربوط به زی پرشین
\usepackage{xepersian}
%\settextfont[Scale=1]{Yas}
%\settextfont{XB Niloofar}
\settextfont{XB Kayhan}
%%%%%%%%%%%%%%%%
\newcounter{exnum}
\newenvironment{exchapter}[1][]{\refstepcounter{
exnum}\par\medskip\noindent%
    \textbf{\theexnum. #1} }{\medskip}
\setlength{\parindent}{0pt}
\newoutputstream{solutions}
\openoutputfile{\jobname.solutions}{solutions}
\newcounter{exblock}
\newcounter{exquestion}
\newcounter{exsubquestion}
\newlist{questionlist}{enumerate}{1}
\newlist{solutionlist}{enumerate}{1}
\newlist{subquestionlist}{enumerate*}{1}
\newlist{subsolutionlist}{enumerate*}{1}
\setlist[questionlist,solutionlist]{label=\arabic*.}
\setlist[subquestionlist,subsolutionlist]{label=(\alph*),itemjoin={},before=~}
\newcommand{\exheader}{%
    \ifthenelse{\value{exblock}>0}{\addtostream{solutions}{\noexpand\end{solutionlist}}}{}%
    \refstepcounter{exblock}%
    \setcounter{exquestion}{0}%
    \section{مسائل}%
    \label{ex.\arabic{exblock}}%
    \addtostream{solutions}{\noexpand\paragraph{مسائل \noexpand\ref{ex.\arabic{exblock}}, صفحه \noexpand\pageref{ex.\arabic{exblock}}}}%
    \addtostream{solutions}{\noexpand\begin{solutionlist}}%
}
\newcommand{\printsolutions}{%
\begin{adjmulticols}{2}{-15.3em}{0pt}
    \ifthenelse{\value{exblock}>0}{\addtostream{solutions}{\noexpand\end{solutionlist}}}{}%
    \closeoutputstream{solutions}%
    \input{\jobname.solutions}%
   \end{adjmulticols}
}
\newenvironment{exercisescol}[2][]{%
    \begin{adjmulticols}{2}{0em}{0pt}%
       \begin{questionlist}[start=\value{exquestion}+1, #1]%
  }{%
    \end{questionlist}%
    \end{adjmulticols}%
}
\newcommand{\question}[2]{%
    \refstepcounter{exquestion}%
    \label{ex.\arabic{exblock}.\arabic{exquestion}}%
    \item #1%
    \addtostream{solutions}{\noexpand\item[\noexpand\ref{ex.\arabic{exblock}.\arabic{exquestion}}.] #2}%
}
\setlist*[questionlist]{font=\bfseries}
\setlist*[solutionlist]{noitemsep,font=\bfseries}
%%%%%%%%%%%%%%%%
\begin{document}
\chapter{فصل اول}
\ptext{1-2}
\exheader
\begin{exercisescol}[noitemsep]{2}
   \question{
$\int\sqrt{x}dx$}{$\tfrac{2}{3}x\sqrt{x}+C$}
    \question{$\int3e^xdx$}{$3e^x+C$}
    \question{$\int(3x^2-\sqrt{5x}+2)dx$}{$x^3-\tfrac{2}{3}x\sqrt{5x}+2x+C$}
    \question{$\int(\tfrac{1}{2x}-\tfrac{2}{x^2}+\tfrac{3}{\sqrt{x}})dx$}{$\tfrac{1}{2}\ln|x|+\tfrac{2}{x}+6\sqrt{x}+C$}
    \question{$\int\sqrt{x}dx$}{$\tfrac{2}{3}x\sqrt{x}+C$}
    \end{exercisescol}
\chapter{پاسخ تمرینات}
\printsolutions
\end{document}