The Key To... series is a very self-instructional, unintimidating, programmed learning series. Covering seven basic topics, students can work through these horizontally or vertically. They are ideal for "homework" or a more topical approach to the subject. Presentation is done in a simple, friendly manner, incrementally, and with examples to follow as needed. Workbooks are thin and consumable. Answer keys are recommended. Order by the book or in sets.
We like this series because it encourages children to tackle these subject areas in as much depth as they can, rather than working strictly within the confines of an age-graded math program. Starting in 4th or 5th grade, I purchased a complete set of student workbooks for each child. Book 1 in each topic was put on the school book shelf. When Key to... was assigned, my children were free to work on whatever topic they chose, and they worked as much as they wanted. As soon as Book 1 in any subject was completed, it was replaced by Book 2. My oldest daughter sometimes got so involved that she just kept going!