This material is a must for any 3-6 or 6-9 Montessori classroom to aid children in the memorization of subtraction facts with linear counting. I have made 5 levels to this set, each level consisting of 15 numbered cards, each card consisting of 5 mixed operation/subtraction problems. Level 1 simply begins with only 2 addends and 1 subtrahend to find the difference for each problem. Each level becomes increasingly more challenging with higher numbers, more digits, and no set addition or subtra