[問題] theorem 的 counter

Is this what you want?

\section{Testing}

\begin{exercise}
Ex1
\end{exercise}

\begin{answer}
Ans1
\end{answer}

\renewcommand{\theanswer}{\theexercise}   <---

\begin{exercise}
Ex2
\end{exercise}

\begin{answer}
Ans2
\end{answer}