Sideways Arithmetic from Wayside School by Louis Sachar

It's the most crazy mixed-up school you've ever seen. You'll meet all the kids you read about in Sideways Stories from Wayside School, plus you'll have the chance to solve over fifty mindboggling math puzzles and brainteasers. Why does elf + elf = fool? How many meals will Miss Mush, the lunch teacher, have to cook for the food to taste as bad as it smells? These puzzles may look tricky, but the answers are perfectly logical. Try solving these brainteasers along with the kids from Mrs. Jewls's class. You'll learn a lot about math - but you'll be laughing too hard to notice.