documentclass{gsm-l}
 usepackage[papersize={160mm,240mm},inner=20mm, outer=15mm, vmargin=15mm]{geometry}
 geometry{includeheadfoot}
 usepackage{amssymb, amsmath}
 setcounter{tocdepth}{2}
 usepackage{pgf,tikz}
 usetikzlibrary{arrows}
 newtheorem{problem}{Problem}
 numberwithin{problem}{chapter}
 begin{document}
 chapter{Introduction}
 begin{problem}
 This is problem 1.1
 end{problem}
 chapter{Key to the exercises}
 I want to know how to auto list the problem number and the solution is behind it,
 like :

noindent textbf{Problem 1.1}

The following is solution.
end{document}