\(n_{Al_2O_3}=\dfrac{20.4}{102}=0.2\left(mol\right)\)
\(n_{H_2SO_4}=\dfrac{17.64}{98}=0.18\left(mol\right)\)
\(Al_2O_3+3H_2SO_4\rightarrow Al_2\left(SO_4\right)_3+3H_2O\)
\(1.............3\)
\(0.2...........0.18\)
\(LTL:\dfrac{0.2}{1}>\dfrac{0.18}{3}\Rightarrow Al_2O_3dư\)
\(m_{Al_2\left(SO_4\right)}_{_3}=0.06\cdot342=20.52\left(g\right)\)