$Mg+2HCl\to MgCl_2+H_2$
$2Al+6HCl\to 2AlCl_3+3H_2$
$2Cu+O_2\buildrel{{t^o}}\over\to 2CuO$
$HCl+NaOH\to NaCl+H_2O$
$AlCl_3+4NaOH\to 3NaCl+NaAlO_2+2H_2O$
$MgCl_2+2NaOH\to Mg(OH)_2+2NaCl$
$Mg(OH)_2\buildrel{{t^o}}\over\to MgO+H_2O$
$n_{CuO}=\dfrac{16}{80}=0,2(mol)$
$\Rightarrow n_{Cu}=n_{CuO}=0,2(mol)$
$\%m_{Cu}=\dfrac{0,2.64.100}{20}=64\%$
$n_{MgO}=\dfrac{8}{40}=0,2(mol)$
$\Rightarrow n_{Mg}=n_{MgO}=0,2(mol)$
$\%m_{Mg}=\dfrac{0,2.24.100}{20}=24\%$
$\Rightarrow \%m_{Al}=12\%$