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解方程组
(1) $\left\{\begin{array}{c}2 x-y=5 \\ 7 x-3 y=20\end{array}\right.$
(2) $\left\{\begin{array}{c}m-\frac{n}{2}=2 \\ 2 m+3 n=12\end{array}\right.$
                        
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