An encrypted clue
(Book)

"When Stephanie Lewis finds secret writing in the margin of an old book in the library, The Math Kids have a new puzzle to solve. But first, they'll have to learn about codes and ciphers and how they can use their math skills to solve them. As one clue leads to another, the kids are drawn into the mysterious old house that overlooks the town. Is it really haunted like some of the townspeople say? And who is the man in the long beard who keeps showing up everywhere they go? But that's not their only problem. The town they live in is broke. Unless they can find a solution, the math competition they've been training so hard for will be cancelled. Jordan, Stephanie, Justin, and Catherine will need to use all their problem-solving skills to figure out the clues before it's too late"--Amazon.com.