http://ntic.educacion.es/w3//recursos/primaria/matematicas/fracciones/menuu1.html
http://phobos.xtec.net/rferna63/mod/resource/view.php?id=1597
http://phobos.xtec.net/rferna63/mod/resource/view.php?id=1577
http://phobos.xtec.net/rferna63/mod/resource/view.php?id=1578
http://phobos.xtec.net/rferna63/mod/resource/view.php?id=1580