Daisy and Dale's father, adventurer Darren Deed, has gone missing during a mission to South America. After the kids receive a mysterious package in the mail, clues unravel that might lead to his whereabouts. He's possibly in the mysterious city of El Numero. Daisy and Dale need readers' help, and math skills, as they attempt to rescue their father. This clever volume seamlessly combines an exciting adventure story with need-to-know math concepts and is packed with appealing illustrations. After solving problems in this fantasy world, readers will find themselves making more math connections in their own lives.