You are given 15 mole of CO. The problem specifies that Co must be reacted to Fe3O4 and one of its products is CO2. The balanced chemical equation is Fe3O4 + 4CO -> 3Fe + 4CO2. Note that for every mole of Fe3O4, four moles of CO is needed to produce four moles of CO2 base on the chemical reaction. To get the mass of CO2, multiply 15 mol CO to the molar ratio of CO2 and CO (4/4).
15 mol CO (4 mol CO2/4 mol CO)(44 g CO2/1 mol CO2) = 660 g CO2