Kenney, Daniel. Math Inspectors #4

"Summer vacation has finally arrived, and the Math Inspectors deserve a break. After all, their sixth-grade year was a busy one. On top of all the normal school stuff, Stanley, Charlotte, Gertie, Felix, and Herman made quite a name for themselves as amateur detectives. But when a relaxing day of roller coasters, riddle booths, and waffle eating contests turns into a desperate scramble to save a beloved landmark, the friends quickly discover that this case may be asking more than they are willing to give. In fact, there may only be one way out--to quit. Will this be the end of the Math Inspectors?"--back cover