Gọi x, y, z là mol $Al$, $Al_2O_3$, $MgO$
$\Rightarrow 27x+102y+40z=40$ (1)
$n_{NaOH}=0,3.2=0,6 mol$
$2Al+2NaOH+2H_2O\to 2NaAlO_2+3H_2$
$Al_2O_3+2NaOH\to 2NaAlO_2+H_2O$
$\Rightarrow x+2y=0,6$ (2)
$n_{H_2}=\dfrac{6,72}{22,4}=0,3 mol$
$\Rightarrow 1,5x=0,3$ (3)
(1)(2)(3)$\Rightarrow x=y=0,2; z=0,355$
$\%m_{Al}=\dfrac{27.0,2.100}{40}=13,5\%$
$\%m_{Al_2O_3}=\dfrac{102.0,2.100}{40}=51\%$
$\%m_{MgO}=35,5\%$