Thanks so much for posting this, the comments are helpful. We are doing the same section and it's been difficult for my 7 year old, so we've been spending extra time on it. He can do it reasonable well with the c-rods and flats, but still needs help with the mental math portion. I have Math Mammoth as well, so I've been pulling out the corresponding worksheets and having him work through those.
Should we simmer in this section for a while or just move on...any suggestions?