\[\begin{array}{l}
{}_{17}^{37}Cl - x\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{}_{17}^{35}Cl - y\\
\left[ \begin{array}{l}
\left\{ \begin{array}{l}
\frac{x}{y} = 3\\
x + y = 100
\end{array} \right.\\
\left\{ \begin{array}{l}
\frac{y}{x} = 3\\
x + y = 100
\end{array} \right.
\end{array} \right. \leftrightarrow \left[ \begin{array}{l}
x = 75,\,\,y = 25\\
x = 25,y = 75
\end{array} \right. \to \left[ \begin{array}{l}
\overline {Cl} = \frac{{37.75 + 35.25}}{{100}} = 36,5\\
\overline {Cl} = \frac{{37.25 + 35.75}}{{100}} = 35,5
\end{array} \right.
\end{array}\]