Frankie Pickle and the Mathematical Menace by Eric Wight
Frankie's facing his greatest challenge yet: his MATH QUIZ. He'd rather live in a world without math, but his quiz has other plans. It transports him to Arithmecca, a world filled with horrible number monsters! Armed with only his pencil, Frankie must learn to divide and conquer if he's going to ace his quiz and defeat THE MATHEMATICAL MENACE.