Professor Ellison's Hard Math books are available on Amazon. Each has both a textbook and a workbook. The links below are PDF copies of the solution manuals for each workbook. The Elementary School solutions are complete. The Middle School solutions are a joint project with Chris Avery.
Hard Math for Elementary School: Answer Key for Workbook
Hard Math for Middle School: Answer Key for Workbook
For those who are interested in learning the material in the Middle School book, high school student Noah Abji has posted an extensive set of video lectures to YouTube. You can find links to them on the IMLEM website.