Among the oxides, how many are mixed oxides?
${\mathrm{Mn}}_3{\mathrm{O}}_4,{\mathrm{PbO}}_2,{\mathrm{Pb}}_3{\mathrm{O}}_4,{\mathrm{Fe}}_3{\mathrm{O}}_4,{\mathrm{BaO}}_2,{\mathrm{Al}}_2{\mathrm{O}}_3$

${\mathrm{Mn}}_3{\mathrm{O}}_4,{\mathrm{Pb}}_3{\mathrm{O}}_4,{\mathrm{Fe}}_3{\mathrm{O}}_4$ are mixed oxides, as there are two oxidation state of Mn in Mn₃O₄ which is +2 and +3, Pb₃O₄ is a mixed oxide of PbO and PbO2, and  there are two oxidation state of Fe in Fe₃O₄ which is +2 and +3