Kenney, Daniel. Math Inspectors #2

"Sixth-graders Stanley, Charlotte, Gertie and Felix did more than just start a detective agency. Using their math skills and their gut instincts, they actually solved a crime the police couldn't crack. Now the Math Inspectors are called in to uncover the identity of a serial criminal named Mr. Jekyll, whose bizarre (and hilarious) pranks cross the line into vandalism. But the deeper the friends delve into the crimes, the more they realize why they were asked to help...and it wasn't because of their detective skills." -- Back cover

Ages 9-12