$a/\%m_{Fe}=\dfrac{56}{232}.100\%=24,14\%$
$b/n_{Fe_3O_4}=2,32/232=0,01mol$
$PTHH : 3Fe+2O_2\overset{t^o}\to Fe_3O_4$
Theo pt :
$n_{O_2}=2.n_{Fe_3O_4}=2.0,01=0,02mol$
$⇒m_{O_2}=0,02.32=0,64g$
$c/2KMnO_4\overset{t^o}\to K_2MnO_4+MnO_2+O_2$
Theo pt :
$n_{KMnO_4}=2.n_{O_2}=2.0,02=0,04mol$
$⇒m_{KMnO_4}=0,04.158=6,32g$