Reaction sequence:
2c(s) + o2(g) -> 2co(g)
fe3o4(s) + 4co(g) -> 3fe(l) + 4co2(g)
According to first equation, 2 moles of carbon produce 2 moles of carbon monoxide. So 1 mole of carbon will produce 1 mol of carbon monoxide (the same number).
According to the second equation, 4 moles of carbon monoxide produce 3 moles of iron. We should make the cross multiplication with those numbers:
4 moles CO/3 moles iron = 1 mol CO/x
x = 1 mol CO*3 moles iron/4 moles CO = 0.75 moles of iron