Dear Sarah,

As you probably know, Lesson 90 is preparation for Lesson 91, where the child discovers for himself how to add 4-digit numbers without the AL abacus. I have found that children making that discovery and doing the six worksheets don't forget it. The beauty of truly understanding is that "maintenance" is rarely an issue.

The topic is revisited again in the next level.

Joan

Thank you for your reply, Dr. Cotter. I am looking forward to learning more with my son when we do Level C. I am very grateful for this curriculum. You seem to understand how children think and learn!