$4Al+3O_2\xrightarrow{t^o}2Al_2O_3$
$n_{O_2}=\dfrac{5,04}{22,4}=0,225(mol)$
Theo PT: $n_{Al_2O_3}=\dfrac{2}{3}n_{O_2}=0,15(mol)$
$\Rightarrow m_{Al_2O_3}=0,15.102=15,3(g)$
$\Rightarrow m_{Al(trong X)}=17,2-15,3=1,9(g)$
Theo PT: $n_{Al}=\dfrac{4}{3}n_{O_2}=0,3(mol)$
$\Rightarrow m_{Al(p/ứ)}=0,3.27=8,1(g)$
$\Rightarrow \Sigma m_{Al}=8,1+1,9=10(g)$
$\to m=10$
$\%m_{Al(trong X)}=\dfrac{1,9}{17,2}.100\%\approx 11,05\%$
$\%m_{Al_2O_3}=100-11,05=88,95\%$