Murphy’s introductory math concept endeavor is tailor-made for the picture book format. In this case readers are asked to find the unknown element in a number sentence, which works along the lines of a simple algebraic equation. Murphy frames his story as a trip to the amusement park in which one of the kids loses his tickets and the other four must donate some of theirs to him. Each kid has 20 tickets and all the rides require a different number of tickets, so the kids have to add up the tickets required for their rides, then add or subtract from 20, meanwhile figuring in the ride they are donating to the ticket-loser. For example, if Alicia wants to take 5 Rhino Rides at 2 tickets each, plus a couple of Monkey Games at a ticket apiece, what unknown number does she need to make 20? As the numbers are relatively small, this can be carried out in the reader’s head (nor does it hurt that Murphy’s explanations are crystal clear). The rides look like they could be a lot of fun too, as depicted in Bjorkman’s whirling, caricaturish artwork. *(Picture book/nonfiction. 7-9)*