We are given with the equation 3CO(g) + Fe2O3(s) to form Fe(s) + 3CO2(g). To balance the given equation, we do elemental balance and verify each. for C we have 3 moles in each side. For O, we have 6 moles in each side. However for Fe, we have 2 moles in the left and only one mole in the right. Hence, there should be 2 moles of Fe(s) in the balance