$$\left({\mathtt{9}}{\mathtt{\,\small\textbf+\,}}{\mathtt{30}}\right){\mathtt{\,\small\textbf+\,}}\left[{\mathtt{4}}{\mathtt{\,-\,}}\left({\mathtt{12}}\right)\right] = {\mathtt{39}}{\mathtt{\,\small\textbf+\,}}\left({\mathtt{4}}{\mathtt{\,-\,}}{\mathtt{12}}\right) = {\mathtt{39}}{\mathtt{\,-\,}}\left({\mathtt{8}}\right) = {\mathtt{39}}{\mathtt{\,-\,}}{\mathtt{8}} = {\mathtt{31}}$$
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