This material is a must for any 3-6 or 6-9 Montessori classroom to aid children in the memorization of single-digit addition facts with linear counting. I have made 5 levels to this set, each level consisting of 15 numbered cards, each card consisting of 5 addition problems. Level 1 simply begins with only 2 addends, but each level becomes increasingly more challenging with higher digits and more addends. After completing 75 cards, and 375 addition problems, a child will have self-confidence