\(n_{KMnO_4}=\dfrac{15.8}{158}=0.1\left(mol\right)\)
\(2KMnO_4\underrightarrow{^{^{t^0}}}K_2MnO_4+MnO_2+O_2\)
\(0.1..............................................0.05\)
\(V_{O_2}=0.05\cdot22.4=1.12\left(l\right)\)
\(V_{O_2\left(tt\right)}=1.12\cdot80\%=0.896\left(l\right)\)