Hi Lisa! If you order the HIG 5A/5B, you will get the Home Instructor Guides, which were written by Sonlight and include further examples and more fully-worked answers. The Singapore Answer Key for 4A-6B is just the answers, without the more fully-worked solutions. I used 6A/6B HIG for help when I needed it. I did not pull it out daily, but found it useful for those difficult multi-step problems, as well as for additional examples. The 5A/5B Textbooks are used to teach the lessons, are written by Singapore, and are paired with the 5A/5B workbooks, working together just as they did for the earlier years in Singapore. HTH!
In Christ,
Julie