\(4Al+3O2-->2Al2O3\)
\(2KMnO4-->K2MnO4+MnO2+O2\)
a)\(n_{Al2O3}=\frac{20,4}{102}=0,2\left(mol\right)\)
\(n_{O2}=\frac{3}{2}n_{Al2O3}=0,3\left(mol\right)\)
\(n_{KMnO4}=2n_{O2}=0,6\left(mol\right)\)
\(m_{KMnO4}=0,6.158=94,8\left(g\right)\)
b)\(n_{Al}=2n_{Al2O3}=0,4\left(mol\right)\)
\(m_{Al}=0,4.27=10,8\left(g\right)\)