Let \(X\) be a set and \(\mu ^*\) be an outer measure on \(X\). A set \(A\subset X\) is called \(\mu ^*\)-measurable if for every \(S\subset X\) we have
\[ \mu ^*(S)=\mu ^*(S\cap A)+\mu ^*(S\setminus A). \]
Show that \(\mu ^*\) is countably additive on the \(\mu ^*\) measurable sets.