Murphy continues to amuse with math-quite a trick considering the amount of anxiety the subject manages to generate. Here he tackles the concept of comparative value by working it into a story of trading cards. Mike and his brother Andy are dinosaur trading card enthusiasts. It's Andy's birthday and as a special treat he is getting to accompany older brother Mike to a trading fair for the first time. Mike is in hot pursuit of a Tyrannosaurus rex card, but he has to wheel and deal to get the cards the owner of the T. rex wants in trade. As the story progresses, small boxed items on the page allow readers to see the action in terms of equations-two Pterosaur cards equals one Stegosaurus card, four of which equal one Tyrannosaurus card-and also learn a few facts about the dinosaurs. And sweetly, at the end of all the furious trading Mike gives Andy the card for his birthday present. O'Malley's color-shocked artwork is a real plus to Murphy's story, which easily takes the mystifying sting out of comparatives. A guide is included to help kids get the most out of the book, and a few games are suggested enjoyably extend the math lesson.