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A couple of my students were having difficulty mastering subtraction with regrouping (a skill they have been practicing since the younger grades). It was clear they did not understand WHEN they should be regrouping and WHY they were doing so. The concept was too abstract and I knew I needed to make it more concrete. Grabbing my bins of base ten blocks, I sat down and started using them to instruct and SHOW the meaning of subtraction. Reviewing the idea of "trading" the tens for ones and hundreds for tens was the first step. I then gave the students a number to 'build' with the base ten models.