$n_{Fe_3O_4}=139,2/232=0,6mol$
$3Fe+2O_2\overset{t^o}\to Fe_3O_4$
$a/$
Theo pt :
$n_{Fe}=3.n_{Fe_3O_4}=2.0,6=1,8mol$
$⇒m_{Fe}=1,8.56=100,8g$
$b/$
Theo pt :
$n_{O_2}=2.n_{Fe_3O_4}=2.0,6=1,2mol$
$⇒V_{O_2}=1,2.22,4=26,88l$
$c/$
$2KMnO_4\overset{t^o}\to K_2MnO_4+MnO_2+O_2$
Theo pt :
$n_{KMnO_4}=2.n_{O_2}=2.1,2=2,4mol$
$⇒m_{KMnO_4}=2,4.158=379,2g$