\(MgO+H_2SO_4-->MgSO_4+H_2O\)
x..............x.........................x.....................x
\(CuO+H_2SO_4-->CuSO_4+H_2O\)
y...............y........................y.......................y
\(C\%_{MgSO_4}=C\%_{CuSO_4}\Rightarrow120x=160y\Rightarrow y=\dfrac{3x}{4}\)
\(\%MgO=\dfrac{40x}{40x+160.\dfrac{3x}{4}}.100\%=\dfrac{40x}{\dfrac{640x}{4}}.100\%=\dfrac{160x}{640x}.100\%=25\%\)
\(\%CuO=100\%-25\%=75\%\)