"Casting nines" is a fun and largely accurate method for students to check their own work solving basic math problems (addition, subtraction, multiplication, division). By adding the numerals in a given problem and subtracting nine from that sum if it is more than nine, students can determine whether they solved the problem correctly and make corrections if necessary. This is more or less a reproducible drill book with solutions included, except that instead of practicing problems students are practicing checking problems. While this can be a useful skill, it's perhaps less necessary in a homeschool situation than it would be in a classroom since parents don't have to have kids check their own work to save time. Still, learning to "cast nines" can be a good way to build confidence, and to stretch students' minds with a little out-of-the-ordinary mathematical theory.