The Elementary series consists of 10 books and is suitable from K to about 4th grade. This series does not work according to specific grades, so the grade suggestions are approximate.
The Intermediate series consists of 3 books, suitable from 4th to 6th grade.
The middle school series should be started from about 6th grade, but not before 5th grade.
Each book in the middle school series has optional practice books:
The high school series consists of 4 books and 2 optional practice books: