$n_{KMnO_4}=15,8/158=0,1mol$
$2KMnO_4\overset{t^o}\to K_2MnO_4+MnO_2+O_2$
Theo pt :
$n_{O_2}=1/2.n_{KMnO_4}=1/2.0,1=0,05mol$
$⇒V=0,05.22,4=1,12l=1120ml$
$80\%n_{O_2}=80\%.0,05=0,04mol$
$2Mg+O_2\overset{t^o}\to 2MgO$
Theo pt :
$n_{Mg}=2.n_{O_2}=2.0,04=0,08mol$
$⇒m_{Mg}=0,08.24=1,92g$