How to make star Exercise and its environment answer?

Question

At here How to make an environment star Exercise?, I posted make star Exercise.

How to make star Exercise and its environmet answer? Example, with Exercise 1.1, we have Answer 1.1, with Exercise 1.5*, we have Answer 1.5*.

Answer 1

The following provides the functionality you requested by defining the sol* environment to be very similar to the ex* environment:

\documentclass{book}
\usepackage{amsfonts,amssymb,amsmath}% http://ctan.org/pkg/{amsfonts,amssymb,amsmath}
\newtheorem{ex}{Exercise}[chapter]
\newenvironment{ex*}
  {\renewcommand\theex{\thechapter.\arabic{ex}\rlap{$^*$}}%
   \ex\edef\@currentlabel{\thechapter.\arabic{ex}}}
  {\endex}
\newtheorem{sol}{Answer}[chapter]
\newenvironment{sol*}
  {\renewcommand\thesol{\theex\rlap{$^*$}}%
   \sol\edef\@currentlabel{\theex}}
  {\endsol}
\renewcommand\thesol{\theex}

\begin{document}
\chapter{First Chapter}
\section{First section}
\begin{ex}
This is an Exercise.
\end{ex}
\begin{sol}
This is a Solution.
\end{sol}

\begin{ex}
This is an Exercise.
\end{ex}

\begin{ex*}
This is a difficult Exercise.
\end{ex*}
\begin{sol*}
This is an easy solution.
\end{sol*}

\end{document}

The base code was taken from egreg's answer.