Karen,

Thank you for your message. Subtraction is introduced in Level B, Lesson 93, and Lesson 102 also does a wonderful job reviewing some of the basic concepts.

In Level C, I would review Lesson 36, Subtraction as Missing Addends, this will help solidify the concept of subtraction. Also in Level C, I would review, if needed, Lessons 37, 38, 39, and 40 would provide great review. In addition the following lessons seem to beneficial to the concept of subtraction; 87, 90, 91, 92, 94, 100, 110 and 150. I would also do the games in lessons 56 and 57, and the bead trading activity in Lesson 87. Another reference is the mid-year test found in the back of the book with problems relating to multiple digit subtraction.

With Level D, subtraction is reviewed throughout the book. If I was in your situation, I would use Level C as a guide for review and re-introduction of the concepts.

I hope this helps you, please let me know if there is anything additional that we can help with!

Make it a great day!

Cassie :)